Communication method, user termnal and processor

ABSTRACT

A communication method according to an embodiment comprises: a step of performing, by a user terminal, a reselection of a target cell to be used as a serving cell from among a plurality of cells operated at different frequencies. The step of performing the reselection of the target cell comprises: a first reselection step of performing, by the user terminal, the reselection of the target cell in a response to reception of a paging signal for requesting the reselection of the target cell from a current serving cell; and a second reselection step of performing, by the user terminal, the reselection of the target cell in a response to a trigger without dependence on the paging signal. In the first reselection step, the user terminal starts measurement of a quality of a measurement target frequency included in system information broadcast from the current serving cell, in response to the reception of the paging signal.

RELATED APPLICATIONS

This application is a continuation application of internationalapplication PCT/JP2016/061616, filed on Apr. 8, 2016, which claimsbenefit of the U.S. Provisional Application No. 62/145,869, filed onApr. 10, 2015, the U.S. Provisional Application No. 62/165,315, filed onMay 22, 2015, the U.S. Provisional Application No. 62/188,873, filed onJul. 6, 2015, the U.S. Provisional Application No. 62/218,689, filed onSep. 15, 2015, the U.S. Provisional Application No. 62/247,948, filed onOct. 29, 2015, and the U.S. Provisional Application No. 62/281,446,filed on Jan. 21, 2016, the entirety of all applications herebyexpressly incorporated by reference.

TECHNICAL FIELD

The present disclosure relates to technology for selecting a target cellused as a serving cell from a plurality of cells operated at differentfrequencies.

BACKGROUND ART

In 3GPP (3rd Generation Partnership Project), which is a project aimingto standardize a mobile communication system, a technology for selectinga target cell used as a serving cell from a plurality of cells operatedat different frequencies is proposed.

Specifically, a user terminal measures, if a start condition issatisfied, a quality of a neighboring cell adjacent to the currentserving cell, and selects, from the cells that satisfy selectionconditions, the target cell used as the serving cell.

SUMMARY

A communication method according to an embodiment comprises: a step ofperforming, by a user terminal, a reselection of a target cell to beused as a serving cell from among a plurality of cells operated atdifferent frequencies. The step of performing the reselection of thetarget cell comprises: a first reselection step of performing, by theuser terminal, the reselection of the target cell in a response toreception of a paging signal for requesting the reselection of thetarget cell from a current serving cell; and a second reselection stepof performing, by the user terminal, the reselection of the target cellin a response to a trigger without dependence on the paging signal. Inthe first reselection step, the user terminal starts measurement of aquality of a measurement target frequency included in system informationbroadcast from the current serving cell, in response to the reception ofthe paging signal.

A user terminal according to an embodiment comprises a controllerconfigured to perform a reselection of a target cell to be used as aserving cell from among a plurality of cells operated at differentfrequencies. The controller is configured to: perform a first process ofperforming the reselection of the target cell in a response to receptionof a paging signal for requesting the reselection of the target cellfrom a current serving cell; and perform a second process of performingthe reselection of the target cell in a response to a trigger withoutdependence on the paging signal. The controller is configured to startmeasurement of a quality of a measurement target frequency included insystem information broadcast from the current serving cell, in responseto the reception of the paging signal.

A processor according to an embodiment is a processor for controlling auser terminal. The processor is configured to execute: a process ofperforming a reselection of a target cell to be used as a serving cellfrom among a plurality of cells operated at different frequencies. Inthe process of performing the reselection of the target cell, theprocessor is configured to execute: a first process of performing thereselection of the target cell in a response to reception of a pagingsignal for requesting the reselection of the target cell from a currentserving cell; and a second process of performing the reselection of thetarget cell in a response to a trigger without dependence on the pagingsignal. In the first process, the processor is configured to execute ofa process of starting measurement of a quality of a measurement targetfrequency included in system information broadcast from the currentserving cell, in response to the reception of the paging signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a configuration diagram of an LTE system according to anembodiment.

FIG. 2 is a block diagram of a UE 100 according to the embodiment.

FIG. 3 is a block diagram of an eNB 200 according to the embodiment.

FIG. 4 is a protocol stack diagram of a radio interface according to theembodiment.

FIG. 5 is a configuration diagram of a radio frame used in the LTEsystem according to the embodiment.

FIG. 6 is a diagram for describing an application scene according to afirst embodiment.

FIG. 7 is a sequence diagram illustrating a mobile communication methodaccording to the first embodiment.

FIG. 8 is a sequence diagram illustrating a mobile communication methodaccording to a first modification.

FIG. 9 is a diagram for describing an application scene according to theembodiment.

FIG. 10 is a sequence diagram illustrating a mobile communication methodaccording to the embodiment.

FIG. 11 is a flow diagram illustrating the mobile communication methodaccording to the first modification.

FIG. 12 is a sequence diagram illustrating a mobile communication methodaccording to a second modification.

FIG. 13 is a diagram illustrating an application scene according to atwentieth modification.

FIGS. 14A to 14C are diagrams according to an additional remark of theembodiment.

FIG. 15 is a diagram according to the additional remark of theembodiment.

FIGS. 16A to 16C are diagrams according to the additional remark of theembodiment.

FIGS. 17A and 17B are diagrams according to the additional remark of theembodiment.

FIG. 18 is a diagram according to the additional remark of theembodiment.

FIG. 19 is a diagram according to the additional remark of theembodiment.

FIG. 20 is a diagram according to the additional remark of theembodiment.

DESCRIPTION OF THE EMBODIMENT

Hereinafter, a mobile communication system according to an embodimentwill be described with reference to the drawings. It is noted that, inthe following description of the drawings, identical or like numeralsand symbols are assigned to identical or like parts.

However, it should be noted that the drawings are schematically shownand the ratio and the like of each dimension may be different from thereal ones. Accordingly, specific dimensions and the like should befinalized in consideration of the explanation below. In addition,needless to say, among the drawings, the dimensional relationship andthe ratio may be different.

[Overview of Disclosure]

In the mobile communication system mentioned in BACKGROUND ART, inselecting the target cell to be used as the serving cell, the load ofeach cell is not taken into consideration. Therefore, load distributionof each cell may not be appropriately performed.

A user terminal according to Overview of Disclosure includes acontroller configured to select a target cell to be used as the servingcell from among a plurality of cells operated at different frequencies,and a receiver configured to receive, from a current serving cell, areselection request signal for requesting reselection of the targetcell. The controller performs, in response to the reception of thereselection request signal, the reselection of the target cell.

A user terminal according to Overview of Disclosure includes acontroller configured to select a target cell to be used as a servingcell from among a plurality of cells operated at different frequencies,and a receiver configured to receive a paging signal broadcast from acurrent serving cell. The controller performs, depending on thereselection request signal included in the paging signal, thereselection of the target cell.

The radio base station according to Overview of Disclosure includes atransmitter configured to broadcast a paging signal at a differentpaging occasion. The transmitter broadcasts the paging signal includinga reselection request signal for requesting reselection of a target cellto be used as a serving cell from among a plurality of cells operated atdifferent frequencies.

A user terminal according to Overview of Disclosure includes acontroller configured to select a target cell to be used as the servingcell from among a plurality of cells operated at different frequencies,and a receiver configured to receive, from a current serving cell, areselection request signal for requesting reselection of the targetcell. The controller performs, in response to a trigger that is withoutdependence on the reselection request signal, a first reselectionprocess, while performing, in response to the reception of thereselection request signal, a second reselection process different fromthe first reselection process.

A user terminal according to Overview of Disclosure includes acontroller configured to select a target cell to be used as a servingcell from among a plurality of cells operated at different frequencies.A measurement target frequency that is a frequency to be measured or tobe selected in reselection of the target cell is included in thebroadcast information that is broadcast from a current serving cell. Thecontroller narrows down the measurement target frequency, based on apriority of the frequency.

A user terminal according to Overview of Disclosure includes acontroller configured to select a target cell to be used as the servingcell from among a plurality of cells operated at different frequencies,and a receiver configured to receive, from a current serving cell, areselection request signal for requesting reselection of the targetcell. If the controller receives, from the current serving cell, are-distribution parameter for distributing the target cell selected asthe serving cell, the controller performs, in response to the receptionof the reselection request signal, a first re-distribution process inwhich the re-distribution parameter is used, and if the controller doesnot receive the re-distribution parameter from the current serving cell,the controller performs, in response to the reception of the reselectionrequest signal, a second re-distribution process in which there-distribution parameter is not used.

A user terminal according to Overview of Disclosure includes acontroller configured to select a target cell to be used as the servingcell from among a plurality of cells operated at different frequencies,and a receiver configured to receive, from a current serving cell, areselection request signal for requesting reselection of the targetcell. In response to the reception of the reselection request signal,the controller performs reselection of the target cell for a cell with atarget frequency, while activating a predetermined timer. The controlleruses the cell with the target frequency as the target cell while thepredetermined timer is running. The controller changes an expiry time ofthe predetermined timer, based on a movement speed of the user terminal.

A user terminal according to Overview of Disclosure includes acontroller configured to select a target cell to be used as the servingcell from among a plurality of cells operated at different frequencies,and a receiver configured to receive, from a current serving cell, areselection request signal for requesting reselection of the targetcell. In response to the reception of the reselection request signal,the controller performs reselection of the target cell for a cell with atarget frequency, while activating a predetermined timer. The controlleruses the cell with the target frequency as the target cell while thepredetermined timer is running. The controller stops the predeterminedtimer upon receiving a stop request signal for requesting stop of thepredetermined timer.

A user terminal according to Overview of Disclosure includes acontroller configured to select a target cell to be used as the servingcell from among a plurality of cells operated at different frequencies,and a receiver configured to receive, from a current serving cell, areselection request signal for requesting reselection of the targetcell. In response to the reception of the reselection request signal,the controller performs reselection of the target cell for a cell with atarget frequency, while activating a predetermined timer. The controlleruses the cell with the target frequency as the target cell while thepredetermined timer is running. The controller performs, in response toreception of an inter-frequency reselection request signal while thepredetermined timer is running, reselection of the target cell for acell with an inter-frequency.

According to a user terminal of Overview of Disclosure, it is possibleto appropriately perform load distribution of each cell. In other words,a user terminal in an RRC idle state can be appropriately distributed toeach cell.

First Embodiment

A first embodiment will be described by using, as an example, an LTEsystem based on the 3GPP standards as a mobile communication system,below.

(System Configuration)

A system configuration of the LTE system according to the firstembodiment will be described below. FIG. 1 is a configuration diagram ofthe LTE system according to the first embodiment.

As illustrated in FIG. 1, the LTE system according to the firstembodiment includes a UE (User Equipment) 100, an E-UTRAN (Evolved-UMTSTerrestrial Radio Access Network) 10, and an EPC (Evolved Packet Core)20.

The UE 100 corresponds to a user terminal. The UE 100 is a mobile typecommunication device, and performs a radio communication with the cell(with the serving cell, if the UE 100 is in the RRC connected state)formed by an eNB 200. The configuration of the UE 100 will be describedlater.

The E-UTRAN 10 corresponds to a radio access network. The E-UTRAN 10includes an eNB 200 (evolved Node-B). The eNB 200 corresponds to a radiobase station. The eNBs 200 are connected mutually via an X2 interface.The configuration of the eNB 200 will be described later.

The eNB 200 forms one or a plurality of cells and performs radiocommunication with the UE 100 which establishes a connection with thecell of the eNB 200. The eNB 200 has a radio resource management (RRM)function, a routing function of user data, a measurement controlfunction for mobility control and scheduling and the like. The “cell” isused as a term indicating a smallest unit of a radio communication area,and is also used as a term indicating a function of performing radiocommunication with the UE 100.

The EPC 20 corresponds to a core network. The EPC 20 includes an MME(Mobility Management Entity)/S-GW (Serving-Gateway) 300. The MMEperforms various types of mobility control and the like for the UE 100.The S-GW performs transfer control of the user data. The MME/S-GW 300 isconnected to the eNB 200 via an S1 interface. It is noted that theE-UTRAN 10 and the EPC 20 constitute a network of the LTE system.

FIG. 2 is a block diagram of the UE 100. As illustrated in FIG. 2, theUE 100 includes a plurality of antennas 101, a radio transceiver 110, auser interface 120, a GNSS (Global Navigation Satellite System) receiverunit 130, a battery 140, a memory 150, and a processor 160. The memory150 and the processor 160 configure a controller. The radio transceiver110 and the processor 160 configure a transmitter and a receiver. The UE100 may not necessarily have the GNSS receiver unit 130. Furthermore,the memory 150 may be integrally formed with the processor 160, and thisset (that is, a chipset) may be called a processor.

The antenna 101 and the radio transceiver 110 are used to transmit andreceive a radio signal. The radio transceiver 110 converts a basebandsignal (transmission signal) output from the processor 160 into theradio signal, and transmits the radio signal from the antenna 101.Furthermore, the radio transceiver 110 converts the radio signalreceived by the antenna 101 into a baseband signal (reception signal),and outputs the baseband signal to the processor 160.

The user interface 120 is an interface with a user carrying the UE 100,and includes, for example, a display, a microphone, a speaker, varioustypes of buttons and the like. The user interface 120 receives anoperation from a user and outputs a signal indicating the content of thereceived operation to the processor 160. The GNSS receiver unit 130receives a GNSS signal in order to obtain location informationindicating a geographical location of the UE 100, and also outputs thereceived GNSS signal to the processor 160. The battery 140 accumulatespower to be supplied to each block of the UE 100.

The memory 150 stores a program executed by the processor 160 andinformation used for a process by the processor 160. The processor 160includes a baseband processor that performs modulation and demodulation,encoding and decoding and the like of the baseband signal and a CPU(Central Processing Unit) that performs various types of processes byexecuting the program stored in the memory 150. The processor 160 mayfurther include a codec that performs encoding and decoding on sound andvideo signals. The processor 160 executes various types of processes andvarious types of communication protocols described later.

FIG. 3 is a block diagram of the eNB 200. As illustrated in FIG. 3, theeNB 200 includes a plurality of antennas 201, a radio transceiver 210, anetwork interface 220, a memory 230, and a processor 240. The memory 230and the processor 240 configure a controller. The radio transceiver 210(and/or the network interface 220) and the processor 240 configure atransmitter and a receiver. Furthermore, the memory 230 may beintegrally formed with the processor 240, and this set (that is, thechipset) may be called a processor.

The antenna 201 and the radio transceiver 210 are used to transmit andreceive the radio signal. The radio transceiver 210 converts thebaseband signal (transmission signal) output from the processor 240 intothe radio signal, and transmits the radio signal from the antenna 201.Furthermore, the radio transceiver 210 converts the radio signalreceived by the antenna 201 into the baseband signal (reception signal),and outputs the baseband signal to the processor 240.

The network interface 220 is connected to the neighboring eNB 200 viathe X2 interface and is connected to the MME/S-GW 300 via the S1interface. The network interface 220 is used in communication performedon the X2 interface and communication performed on the S1 interface.

The memory 230 stores a program executed by the processor 240 andinformation used for a process by the processor 240. The processor 240includes a baseband processor that performs modulation and demodulation,encoding and decoding and the like of the baseband signal and a CPU thatperforms various types of processes by executing the program stored inthe memory 230. The processor 240 executes various types of processesand various types of communication protocols described later.

FIG. 4 is a protocol stack diagram of a radio interface in the LTEsystem. As illustrated in FIG. 4, the radio interface protocol isclassified into a first layer to a third layer of an OSI referencemodel, such that the first layer is a physical (PHY) layer. The secondlayer includes a MAC (Medium Access Control) layer, an RLC (Radio LinkControl) layer, and a PDCP (Packet Data Convergence Protocol) layer. Thethird layer includes an RRC (Radio Resource Control) layer.

The physical layer performs coding and decoding, modulation anddemodulation, antenna mapping and demapping, and resource mapping anddemapping. Between the physical layer of the UE 100 and the physicallayer of the eNB 200, user data and control information are transmittedvia a physical channel.

The MAC layer performs priority control of data, a retransmissionprocess by a hybrid ARQ (HARQ), a random access procedure, and the like.Between the MAC layer of the UE 100 and the MAC layer of the eNB 200,user data and control information are transmitted via a transportchannel. The MAC layer of the eNB 200 includes a scheduler fordetermining a transport format (a transport block size and a modulationand coding scheme (MCS)) of an uplink and a downlink, and an allocationresource block for the UE 100.

The RLC layer uses functions of the MAC layer and the physical layer totransmit data to the RLC layer of a reception side. Between the RLClayer of the UE 100 and the RLC layer of the eNB 200, user data andcontrol information are transmitted via a logical channel.

The PDCP layer performs header compression and decompression, andencryption and decryption. Further, it should be noted that in the PDCPlayer, a transmission entity for transmitting a data unit (PDCP PDU) ora reception entity for receiving the data unit (PDCP PDU) is formed.

The RRC layer is defined only in a control plane that handles controlinformation. Between the RRC layer of the UE 100 and the RRC layer ofthe eNB 200, control information (an RRC message) for various types ofsettings is transmitted. The RRC layer controls the logical channel, thetransport channel, and the physical channel in response to theestablishment, re-establishment, and release of a radio bearer. If thereis a connection (an RRC connection) between the RRC of the UE 100 andthe RRC of the eNB 200, the UE 100 is in the RRC connected state, andwhen there is no connection (the RRC connection) between the RRC of theUE 100 and the RRC of the eNB 200, the UE 100 is in the RRC idle state.

An NAS (Non-Access Stratum) layer positioned above the RRC layerperforms session management, mobility management, and the like.

FIG. 5 is a configuration diagram of a radio frame used in the LTEsystem. In the LTE system, OFDMA (Orthogonal Frequency DivisionMultiplexing Access) is applied to a downlink, and SC-FDMA (SingleCarrier Frequency Division Multiple Access) is applied to an uplink,respectively.

As illustrated in FIG. 5, the radio frame is configured by 10 subframesarranged in a time direction. Each subframe is configured by two slotsarranged in the time direction. Each subframe has a length of 1 ms andeach slot has a length of 0.5 ms. Each subframe includes a plurality ofresource blocks (RBs) in a frequency direction, and a plurality ofsymbols in the time direction. Each resource block includes a pluralityof subcarriers in the frequency direction. One resource element (RE) isconfigured by one symbol and one subcarrier. Furthermore, of the radioresources (time-frequency resources) allocated to the UE 100, it ispossible to designate a frequency resource by a resource block, anddesignate a time resource by a subframe (or a slot).

(Application Scene)

An application scene will be described, below. FIG. 6 is a diagram fordescribing the application scene according to the first embodiment.

As illustrated in FIG. 6, a plurality of eNBs 200 (for example, an eNB200 #1, an eNB 200 #2, an eNB 200 #3, and an eNB 200 #4) are provided.The eNB 200 #1 has a cell #1 as the radio communication area, the eNB200 #2 has a cell #2 as the radio communication area, the eNB 200 #3 hasa cell #3-1 and a cell #3-2 as the radio communication area, and the eNB200 #4 has a cell #4-1 and a cell #4-2 as the radio communication area.

It is noted that the cell #2, the cell #3-1, the cell #3-2, the cell#4-1, and the cell #4-2 overlap with the cell #1. Further, each cell isoperated at respectively different frequencies. A priority is definedfor the frequencies where each cell is operated. The correspondencerelationship between the frequency and the priority is included insystem information (SIB; System Information Block) broadcast from theeNB 200.

Under such a premise, the UE 100 selects a target cell used as a servingcell from the plurality of cells operated at different frequencies.Specifically, the UE 100 measures, if start conditions are satisfied,the quality of the neighboring cell adjacent to the current servingcell, and selects, from the cells that satisfies selection conditions,the target cell used as the serving cell.

Firstly, the start conditions are shown as follows:

(A1) A frequency having a higher priority than the priority of thefrequency of the current serving cell

-   -   the UE 100 always measures the quality of the frequency having        high priority.

(A2) A frequency having a priority equal to or lower than the priorityof the frequency of the current serving cell

-   -   the UE 100 measures, if the quality of the current serving cell        falls below a predetermined threshold value, the quality of the        frequency having equal priority or low priority.

Secondly, the selection conditions are shown as follows:

(B1) The priority of the frequency of the neighboring cell is higherthan the priority of the current serving cell

-   -   the UE 100 selects a cell that satisfies a relationship of        Squal>Thresh_(X, HighQ) over a predetermined period        (Treselection_(RAT)), or a cell that satisfies a relationship of        Srxlev>Thresh_(X, HighP) over the predetermined period        (Treselection_(RAT)) as the target cell. In such a case, such        criteria to be satisfied by the neighboring cell is sometimes        referred to as “S-criteria”.

Squal represents a cell selection quality level, and is calculated bySqual=Q_(qualmeas) (Q_(qualmin)−Q_(qualminoffset))−Qoffset_(temp).Q_(qualmeas) is a quality level (RSRQ) of the neighboring cell,Q_(qualmin) is a minimum request quality level, Q_(qualminoffset) is apredetermined offset regularly applied to the neighboring cell, andQoffset_(temp) is an offset temporarily applied to the neighboring cell.Thresh_(X, HighQ) is the predetermined threshold value.

Further, Srxlev represents a cell selection reception level, and iscalculated bySrxlev=Q_(rxlevmeas)−(Q_(rxlevmin)−Q_(rxlevminoffset))−Pcompensation−Qoffset_(temp).Q_(rxlevmeas) is a reception level (RSRP) of the neighboring cell,Q_(rxlevmin) is a minimum request reception level, Q_(rxlevminoffset) isthe predetermined offset regularly applied to the neighboring cell,Pcompensation is a parameter related to an uplink capability, andQoffset_(temp) is the offset temporarily applied to the neighboringcell. Thresh_(X, HighP) is the predetermined threshold value.

(B2) The priority of the frequency of the neighboring cell is the sameas the priority of the current serving cell

-   -   UE 100 calculates a ranking R_(s) of the current serving cell        and a ranking R_(u) of the neighboring cell, and selects, as the        target cell, the cell having the higher ranking R_(u) than the        R_(s) over the predetermined period (Treselection_(RAT)). In        such a case, such criteria to be satisfied by the neighboring        cell is sometimes referred to as “R-criteria”.

R_(s) is calculated by R_(s)=Q_(meas,s)+Q_(Hyst)−Qoffset_(temp). R_(n)is calculated by R_(n)=Q_(meas,n)−Q_(offset)−Qoffset_(temp). Q_(meas,s)is the reception level (RSRP) of the current serving cell, andQ_(meas,n) is the reception level (RSRP) of the neighboring cell.Q_(Hyst) is a hysteresis value for facilitating reselection of thecurrent serving cell as the target cell. Qoffset_(temp) is the offsettemporarily applied to the current serving cell and the neighboringcell.

(B3) The priority of the frequency of the neighboring cell is lower thanthe priority of the current serving cell

-   -   The UE 100 selects, under a premise that        Squal<Thresh_(Serving,LowQ) is satisfied over the predetermined        period (Treselection_(RAT)), or Srxlev<Thresh_(Serving,LowP) is        satisfied over the predetermined period (Treselection_(RAT)),        the target cell from the neighboring cells by a similar method        as the above-described (B1).

Thresh_(Serving,LowQ) and Thresh_(Serving,LowP) are the predeterminedthreshold values similarly to Thresh_(X, HighQ) and Thresh_(X, HighP).

It is noted that, various types of parameters used for selecting thetarget cell is included in the broadcast information (SIB; SystemInformation Block) broadcast from the eNB 200. The various types ofparameters include the priority of the frequency(cellReselectionPriority), the predetermined period(Treselection_(RAT)), the various types of offsets (Q_(qualminoffset),Q_(rxlevminoffset), Qoffset_(temp), Q_(Hyst), Qoffset), and the varioustypes of threshold values (Thresh_(X,HighQ), Thresh_(X,HighP),Thresh_(Serving,LowQ), Thresh_(Serving,LowP)).

In the first embodiment, the UE 100 (controller) selects a target cellto be used as a serving cell from among a plurality of cells operated atdifferent frequencies. Specifically, the UE 100 (receiver) receives,from the current serving cell, a reselection request signal forrequesting reselection of the target cell. The UE 100 (controller)performs, in response to the reception of the reselection requestsignal, the reselection of the target cell. Here, it should be notedthat even if the start conditions indicated in the above-described (A1)and (A2) are not satisfied, the UE 100 (controller) starts the qualitymeasurement of a neighboring cell accompanied with the reselection ofthe target cell.

Here, if the load of the current serving cell is equal to or greaterthan a predetermined load, the current serving cell broadcasts thereselection request signal. It is preferable that the current servingcell repeatedly broadcasts the reselection request signal in a periodduring which the load of the current serving cell is equal to or greaterthan the predetermined load. In other words, if the load of the currentserving cell is equal to or greater than the predetermined load, the UE100 (receiver) receives the reselection request signal from the currentserving cell. It is preferable that the UE 100 (receiver) repeatedlyreceives the reselection request signal in the period during which theload of the current serving cell is equal to or greater than thepredetermined load.

In the first embodiment, the reselection request signal includes areselection parameter for making the current serving cell hard to beselected as the target cell. The UE 100 (controller) selects the targetcell, based on the reselection parameter. In particular, the reselectionparameter may be a parameter that designates to change the priority ofthe frequency (cellReselectionPriority) of the current serving cell tothe lowest priority, or an offset indicating the number of steps bywhich the priority of the frequency (cellReselectionPriority) of thecurrent serving cell is lowered. Alternatively, the reselectionparameter may be various types of offset (Qqualminoffset,Qrxlevminoffset, Qoffsettemp, QHyst, Qoffset), and various types ofthreshold value (ThreshX, HighQ, ThreshX, HighP, ThreshServing, LowQ,ThreshServing, LowP).

In the first embodiment, the reselection parameter applied in responseto the reception of the reselection request signal is not included inthe reselection request signal, but may be included in the systeminformation (SIB; System Information Block) broadcast separately fromthe reselection request signal, from the current serving cell.Alternatively, the reselection parameter applied in response to thereception of the reselection request signal may be predetermined. Insuch a case, the UE 100 (controller) performs, in response to thereception of the reselection request signal, the reselection of thetarget cell, based on the reselection parameter broadcast from thecurrent serving cell or the predetermined reselection parameter.

In the first embodiment, even if the reselection request signal isreceived, the UE 100 may maintain existence in the current serving cellwithout performing reselection of the target cell if the predeterminedcondition is satisfied. The predetermined condition is a conditionrelated to at least either item of information of the class of the UE100 (the UE class), the priority of the frequency of the current servingcell, a power consumption setting of the UE 100, a time elapsed sincereceiving the last reselection request signal, and a value havingrandomness.

For example, if the class of the UE 100 (the UE class) is MTC (MachineType Communication) or a data exclusive terminal, a communication dataamount is small, and hence, the UE 100 (controller) preferably maintainsthe existence in the current serving cell without performing reselectionof the target cell.

Alternatively, if the priority of the frequency of the current servingcell is the highest priority, there is a high possibility of receivingMBMS data or providing a D2D proximity service (D2D ProSe), and hence,the UE 100 (controller) preferably maintains the existence in thecurrent serving cell without performing the reselection of the targetcell.

Alternatively, if the power consumption setting of the UE 100 is a lowpower consumption setting, then in order to suppress the powerconsumption, the UE 100 (controller) preferably maintains the existencein the current serving cell without performing reselection of the targetcell.

Alternatively, if the time elapsed since receiving the last reselectionrequest signal has not elapsed a predetermined time, then in order tosuppress a ping-pong phenomenon, the UE 100 (controller) preferablymaintains the existence in the current serving cell without performingreselection of the target cell. For example, the UE 100 (controller)activates a timer at a timing of receiving the last reselection requestsignal, and preferably does not perform reselection of the target celluntil the timer expires.

Alternatively, in order to suppress a situation where the plurality ofUEs 100 perform reselection of the target cell all together and torandomly distribute the UEs 100 to each cell, if a value havingrandomness is a value other than a predetermined value, the UE 100(controller) preferably maintains the existence in the current servingcell without performing reselection of the target cell. The value havingrandomness may not be a value common to all UEs 100 that exist in thecurrent serving cell. For example, the value having randomness may bethe subframe number (SFN) at which the UE 100 receives the reselectionrequest signal, the identifier (UE-ID) of the UE 100 which receives thereselection request signal, or a random number generated by the UE 100.For example, if SFNmodUE−ID (÷n)=0 is satisfied, the UE 100 (controller)performs reselection of the target cell, and if SFNmodUE−ID (÷n)=0 isnot satisfied, the UE 100 does not perform reselection of the targetcell. However, n is a predetermined value or a value broadcast from theserving cell.

(Mobile Communication Method)

A mobile communication method according to the first embodiment will bedescribed below. FIG. 7 is a sequence diagram illustrating the mobilecommunication method according to the first embodiment.

As illustrated in FIG. 7, in step S10, the UE 100 is in the RRC idlestate.

In step S11, the UE 100 receives a reselection request signal broadcastfrom the eNB 200 (the current serving cell).

In step S12, the UE 100 performs, in response to the reception of thereselection request signal, reselection of the target cell. Here, itshould be noted that even if the start conditions indicated in theabove-described (A1) and (A2) are not satisfied, the UE 100 starts thequality measurement of a neighboring cell accompanied with thereselection of the target cell. Information included in the reselectionrequest signal and a method of reselecting the target cell upon thereception of the reselection request signal are as described above.

(Operation and Effect)

The UE 100 according to the first embodiment performs, in response tothe reception of the reselection request signal, the reselection of thetarget cell. That is, in response to a trigger on the network (thecurrent serving cell) side, the UE 100 in the RRC idle state is promptedto perform the reselection of the target cell. Therefore, loaddistribution of each cell can be appropriately performed. In otherwords, the UE 100 in the RRC idle state can be appropriately distributedto each cell.

First Modification

A first modification of the first embodiment will be described below.Differences from the first embodiment will be mainly described below.

Specifically, in the first embodiment, the UE 100 performs, in responseto the reception of the reselection request signal, the reselection ofthe target cell. In contrast, in the first modification, the UE 100(receiver) receives, from the current serving cell, a load informationsignal including the information for designating a cell load. The UE 100(controller) performs, in response to reception of the load informationsignal, the reselection of the target cell. Here, it should be notedthat even if the start conditions indicated in the above-described (A1)and (A2) are not satisfied, the UE 100 (controller) starts the qualitymeasurement of a neighboring cell accompanied with the reselection ofthe target cell.

Here, the load information signal may include information directlyindicating the cell load. Information directly indicating the cell loadmay be, for example, a value that represents the cell load in %.Alternatively, information directly indicating the cell load may be anindex indicating the level of the cell load from among a plurality ofstages of levels.

The load information signal may include information indirectlyindicating the cell load. Information indirectly indicating the cellload may be an offset that is temporarily set to the cell (Qoffsettemp),or a threshold value for determining whether or not to switch from acoverage area of the mobile communication system to a coverage area of awireless LAN (ThreshServingOffloadWLAN, LowP, ThreshServingOffloadWLAN,LowQ). These values are determined depending on the cell load, andtherefore, the UE 100 (controller) can estimate the cell load, based onthese values. Thus, if the current serving cell broadcasts a loadinformation signal including information indirectly indicating the cellload, the current serving cell may not broadcast information directlyindicating the cell load.

In addition to information for designating the cell load of the currentserving cell, the load information signal may be information indicatinga difference (relative value) between the cell load of the currentserving cell and the cell load of a neighboring cell adjacent to thecurrent serving cell. Further, in addition to information fordesignating the cell load of the current serving cell, the loadinformation signal may include information for designating the cell loadof a neighboring cell adjacent to the current serving cell.

In the first modification, the UE 100 (controller) may determine whetheror not to perform reselection of the target cell, based on theinformation included in the load information signal (at least either oneof the cell load of the current serving cell or the cell load of theneighboring cell). The UE 100 (controller) may, for example, performreselection of the target cell if SFNmod (UE-ID÷{(100%−cellload)×100})=0 is satisfied. However, the SFN is a subframe number atwhich the UE 100 receives the load information signal, the UE-ID is anidentifier of the UE 100 which receives the reselection request signal,and the cell load is a value representing the cell load in %.Alternatively, the UE 100 (controller) may perform cell evaluationaccompanied with the reselection of the target cell, after correctingvarious types of offset (Qqualminoffset, Qrxlevminoffset, Qoffset)depending on the cell load. The various types of offset are calculated,for example, by offset=(default offset)×(cell load).

In the first modification, the UE 100 (receiver) may receive, inaddition to the current serving cell, the load information signal fromthe neighboring cell adjacent to the current serving cell. The UE 100(controller) performs reselection of the target cell, based oninformation included in the load information signal received from thecurrent serving cell and the neighboring cell. For example, the UE 100(controller) corrects an offset to be applied to each cell, based on thecell load of each cell.

If the UE 100 (controller) is not able to receive the load informationsignal from a predetermined cell, the UE 100 may perform reselection ofthe target cell by considering the cell load of the predetermined cellas zero.

Even if the UE 100 (controller) receives the load information signal,the UE 100 may maintain existence in the current serving cell withoutperforming reselection of the target cell if the predetermined conditionis satisfied. The predetermined condition is the same as that in thefirst first embodiment.

(Mobile Communication Method)

Hereinafter, the mobile communication method according to the firstmodification will be described. FIG. 8 is a sequence diagramillustrating the mobile communication method according to the firstmodification.

As illustrated in FIG. 8, in step S20, the UE 100 is in the RRC idlestate.

In step S21, the UE 100 receives a load information signal broadcastfrom the eNB 200 (the current serving cell).

In step S22, the UE 100 performs, in response to the reception of theload information signal, reselection of the target cell. Here, it shouldbe noted that even if the start conditions indicated in theabove-described (A1) and (A2) are not satisfied, the UE 100 starts thequality measurement of a neighboring cell accompanied with thereselection of the target cell. Information included in the reselectionrequest signal and a method of reselecting the target cell upon thereception of the reselection request signal are as described above.

(Operation and Effect)

The UE 100 according to the first modification performs, in response tothe reception of the load information signal, the reselection of thetarget cell. That is, in response to a trigger on the network (thecurrent serving cell) side, the UE 100 in the RRC idle state is promptedto perform the reselection of the target cell. Therefore, loaddistribution of each cell can be appropriately performed. In otherwords, the UE 100 in the RRC idle state can be appropriately distributedto each cell.

Other Modifications of First Embodiment

Although not particularly mentioned in the first embodiment, theinformation for designating the cell load may be information indicatingthe load of the current serving cell, or information indicating the loadof the frequency of the current serving cell. The cell load may be theaverage value or the maximum value of the load of a neighboring celloperated in the same frequency as the frequency of the current servingcell.

Although not particularly mentioned in the first embodiment, the loadinformation signal may include a reselection parameter determined by thecell load (the load of the cell, or the load of the frequency of thecell). The UE 100 (controller) selects the target cell to be used as theserving cell, based on the reselection parameter. In particular, thereselection parameter is, as the load of a cell increases, a parameterfor making the cell hard to be selected as the target cell. Furthermore,the reselection parameter may be set based on the cell load of thecurrent serving cell and the cell load of the neighboring cell.

Although not particularly mentioned in the first embodiment, a programmay be provided for causing a computer to execute each process performedby the UE 100 and the eNB 200. Furthermore, the program may be recordedon a computer-readable medium. If the computer-readable medium is used,it is possible to install the program in a computer. Here, thecomputer-readable medium recording therein the program may be anon-transitory recording medium. The non-transitory recording medium mayinclude, but not be limited to, for example, a recording medium such asa CD-ROM or a DVD-ROM.

Alternatively, a chip may be provided which includes: a memory in whicha program for performing each process performed by the UE 100 and theeNB 200 is stored; and a processor for executing the program stored inthe memory.

In the first embodiment, an LTE system was described as an example of amobile communication system. However, the first embodiment is notlimited thereto. The mobile communication system may be a system otherthan the LTE system.

Second Embodiment

A second embodiment will be described by using, as an example, an LTEsystem based on the 3GPP standards as a mobile communication system,below.

(Application Scene)

An application scene will be described below. FIG. 9 is a diagram fordescribing an application scene according to the second embodiment.

As illustrated in FIG. 9, a plurality of eNBs 200 (for example, an eNB200#1, an eNB 200#2, and eNB 200#3, and an eNB 200#4) are provided. TheeNB 200#1 has a cell #1 as a radio communication area, the eNB 200#2 hasa cell #2 as a radio communication area, the eNB 200#3 has a cell #3-1and a cell #3-2 as a radio communication area, and the eNB 200#4 has acell #4-1 and a cell #4-2 as a radio communication area.

It is noted that the cell #2, the cell #3-1, the cell #3-2, the cell#4-1, and the cell #4-2 overlap with the cell #1. Further, each cell isoperated at respectively different frequencies. A priority is definedfor the frequencies where each cell is operated. The correspondencerelationship between the frequency and the priority is included insystem information (SIB; System Information Block) broadcast from theeNB 200.

Under such a premise, the UE 100 selects a target cell to be used as aserving cell from among a plurality of cells operated at differentfrequencies. Specifically, if a start condition is satisfied, the UE 100measures the quality of a neighboring cell adjacent to the currentserving cell, and selects, from among cells that satisfy the selectionconditions, a target cell to be used as the serving cell.

Firstly, the start conditions are shown as follows:

(A1) A frequency having a higher priority than the priority of thefrequency of the current serving cell

-   -   the UE 100 always measures the quality of the frequency having a        high priority.

(A2) A frequency having a priority equal to or lower than the priorityof the frequency of the current serving cell

-   -   the UE 100 measures, if the quality of the current serving cell        falls below a predetermined threshold value, the quality of the        frequency having an equal priority or a low priority.

Secondly, the selection conditions are shown as follows:

(B1) The priority of the frequency of the neighboring cell is higherthan the priority of the current serving cell

-   -   the UE 100 selects a cell that satisfies a relationship of        Squal>ThreshX, HighQ over a predetermined period        (TreselectionRAT), or a cell that satisfies a relationship of        Srxlev>ThreshX, HighP over the predetermined period        (TreselectionRAT) as the target cell. In such a case, a criteria        to be satisfied by the neighboring cell is sometimes referred to        as “S-criteria”.

However, Squal represents a cell selection quality level, and iscalculated by Squal=Qqualmeas−(Qqualmin+Qqualminoffset)−Qoffsettemp.Qqualmeas is a quality level (RSRQ) of the neighboring cell, Qqualmin isa minimum required quality level, Qqualminoffset is a predeterminedoffset steadily applied to the neighboring cell, and Qoffsettemp is anoffset temporarily applied to the neighboring cell. ThreshX, HighQ is apredetermined threshold value.

Further, Srxlev represents a cell selection reception level, and iscalculated bySrxlev=Qrxlevmeas−(Qrxlevmin+Qrxlevminoffset)−Pcompensation−Qoffsettemp.Qrxlevmeas is a reception level (RSRP) of the neighboring cell,Qrxlevmin is a minimum required reception level, Qrxlevminoffset is apredetermined offset steadily applied to the neighboring cell,Pcompensation is a parameter related to uplink capability, andQoffsettemp is an offset temporarily applied to the neighboring cell.ThreshX, HighP are predetermined threshold values.

(B2) The priority of the frequency of the neighboring cell is the sameas the priority of the current serving cell

-   -   The UE 100 calculates a ranking Rs of the current serving cell        and a ranking Rn of the neighboring cell, and selects a cell        having a higher ranking Rn than Rs over a predetermined period        (TreselectionRAT) as the target cell. In such a case, a criteria        to be satisfied by the neighboring cell is sometimes referred to        as “R-criteria”.

However, Rs is calculated by Rs=Qmeas, s+QHyst−Qoffsettemp. Rn iscalculated by Rn=Qmeas, n−Qoffset−Qoffsettemp. Qmeas, s is the receptionlevel (RSRP) of the current serving cell, and Qmeas, n is the receptionlevel (RSRP) of the neighboring cell. QHyst is a hysteresis value forfacilitating reselection of the current serving cell as the target cell.Qoffsettemp is an offset temporarily applied to the current serving celland the neighboring cell.

(B3) The priority of the frequency of the neighboring cell is lower thanthe priority of the current serving cell

-   -   The UE 100 selects, under a premise that Squal<ThreshServing,        LowQ is satisfied over a predetermined period (TreselectionRAT),        or Srxlev<ThreshServing, LowP is satisfied over the        predetermined period (TreselectionRAT), the target cell from        neighboring cells by a method similar to above-described (B1).

However, ThreshServing, LowQ and ThreshServing, LowP are predeterminedvalues similarly to ThreshX, HighQ and Thresh X, HighP.

It is noted that various types of parameter used for selecting thetarget cell are included in the system information (SIB; SystemInformation Block) broadcast from the eNB 200. The various types ofparameter include the priority of the frequency(cellReselectionPriority), a predetermined period (TreselectionRAT),various types of offset (Qqualminoffset, Qrxlevminoffset, Qoffsettemp,QHyst, Qoffset), and various types of threshold value (ThreshX, HighQ,ThreshX, HighP, ThreshServing, LowQ, ThreshServing, LowP).

In the second embodiment, the UE 100 (controller) selects the targetcell used as the serving cell from the plurality of cells operated atdifferent frequencies. Specifically, the UE 100 (controller) measures,even if the quality of the current serving cell satisfies thepredetermined quality criteria, at a predetermined timing, the qualityof the neighboring cell adjacent to the current serving cell. Here,“satisfying the predetermined quality criteria” means that theabove-described start conditions are not satisfied. More particularly,the predetermined quality criteria are, as described in theabove-described (A2), a condition that the quality of the currentserving cell does not fall below the predetermined threshold value.

In other words, the UE 100 (controller) according to the secondembodiment is configured, in principle, if the quality of the currentserving cell satisfies the predetermined quality criteria, not to startquality measurement of the neighboring cell adjacent to the currentserving cell. However, it should be noted that the UE 100 (controller)exceptionally starts the quality measurement of the neighboring cellbased on the system information received from the current serving cell,as described later.

Here, the UE 100 (controller) designates the predetermined timing basedon the system information (SIB; System Information Block) received fromthe current serving cell. It is noted that, the predetermined timingmeans a timing at which the quality measurement of the neighboring cellis started upon selection of the target cell.

The system information may mean to immediately start the qualitymeasurement of the neighboring cell. In such a case, the UE 100(controller) designates the timing at which the system information isreceived as the predetermined timing, and starts the quality measurementof the neighboring cell in response to reception of the systeminformation.

Alternatively, the system information may include a subframe number atwhich the quality measurement of the neighboring cell should be started.The UE 100 (controller) designates the subframe number included in thesystem information as the predetermined timing, and starts the qualitymeasurement of the neighboring cell at the designated subframe number.

Alternatively, the system information may include information indicatinga period of the predetermined timing. For example, the systeminformation includes a timer value to be set in a timer activated at thetiming when the quality measurement of the neighboring cell isterminated. The UE 100 (controller) designates the timing at which thetimer in which the timer value is set is expired, as the predeterminedtiming, and starts the quality measurement of the neighboring cell atthe designated timing.

Here, in a case where the system information includes the informationindicating the period of the predetermined timing, the UE 100(controller) may correct the period of the predetermined timing based ona unique value to the UE 100. The unique value to the UE 100 may be, forexample, the subframe number for receiving the system information, or anidentifier (UE-ID) of the UE 100. The UE 100 (controller) sets a valueobtained by “timer value×UE-ID” to the timer, and starts the qualitymeasurement of the neighboring cell at the timing at which the timer hasexpired.

Alternatively, the system information may include a threshold value tobe compared with a random number generated by the UE 100 (controller).The UE 100 (controller) measures the quality of the neighboring cellbased on a comparison result between the random number and the thresholdvalue at the predetermined timing. For example, the UE 100 (controller)starts, if, at the predetermined timing, a condition such as the randomnumber being larger than the threshold value or the random number beingsmaller than the threshold value is satisfied, the quality measurementof the neighboring cell.

(Mobile Communication Method)

Hereinafter, the mobile communication method according to the secondembodiment will be described. FIG. 10 is a sequence diagram illustratingthe mobile communication method according to the second embodiment.

As illustrated in FIG. 10, in step S10, the UE 100 is in the RRC idlestate.

In step S11, the UE 100 receives the system information (SIB; SystemInformation Block) broadcast from the eNB 200 (the current servingcell).

In step S12, the UE 100 measures, even if the quality of the currentserving cell satisfies the predetermined quality criteria, at thepredetermined timing, the quality of the neighboring cell adjacent tothe current serving cell. The UE 100 designates the predetermined timingbased on the system information (SIB; System Information Block).

(Operation and Effect)

The UE 100 (controller) according to the second embodiment measures,even if the quality of the current serving cell satisfies thepredetermined quality criteria, at the predetermined timing, the qualityof the neighboring cell adjacent to the current serving cell. In otherwords, even for the UEs 100 that are present in the geographically closelocation, the timings at which the quality measurement of theneighboring cell is started upon selection of the target cell aretemporally discrete. Therefore, load distribution of each cell can beappropriately performed. In other words, the UE 100 in the RRC idlestate can be appropriately distributed to each cell.

First Modification

A first modification of the second embodiment will be described, below.Description proceeds with a focus on differences from the secondembodiment, below.

Specifically, in the second embodiment, the UE 100 measures, even if thequality of the current serving cell satisfies the predetermined qualitycriteria, at the predetermined timing, the quality of the neighboringcell adjacent to the current serving cell. On the contrary, in the firstmodification, the UE 100 (controller) selects, as illustrated in (B1) to(B3) described in the second embodiment, the target cell used as theserving cell from the cells having the quality that satisfies thepredetermined quality criteria (S-criteria or R-criteria). In theselection of the target cell, the UE 100 (controller) uses a valuehaving randomness to select the target cell.

The value having randomness may be a value which is not common to allUEs 100 that exist in the current serving cell. For example, the valuehaving randomness is one or more values selected from the identifier(UE-ID) assigned to the UE 100, the random number generated by the UE100 (controller), and an access class (AC) related to an accessregulation of the UE 100.

Here, the UE 100 (controller) may correct the quality (for example,Squal, Srxlev, Q_(meas,s), Q_(meas,n)) based on the value havingrandomness (for example, the UE-ID, the random number, and the AC). Forexample, as a correction method of the quality, new values may bedefined as various types of offsets (Q_(qualminoffset),Q_(rxlevminoffset), Qoffset_(temp), Q_(Hyst), Qoffset), or a new offsetmay be introduced. Various types of offsets are calculated, for example,by offset=(default offset)×(UE-ID÷n). n is a predetermined value or avalue broadcast from the serving cell.

Alternatively, the UE 100 (controller) may correct, based on the valuehaving randomness (for example, the UE-ID, the random number, and theAC), the priority (cellReselectionPriority) of the frequency in whicheach of the plurality of cells is operated.

Alternatively, the UE 100 (controller) may select, based on the valuehaving randomness (for example, the UE-ID, the random number, and theAC), the target cell from the cells having the quality that satisfiesthe predetermined quality criteria (S-criteria or R-criteria). In otherwords, the UE 100 (controller) identifies the cells having the qualitythat satisfies the predetermined quality criteria (S-criteria orR-criteria), and selects, based on the value having randomness, thetarget cell from the identified cells (selection candidate cells). Forexample, the UE 100 (controller) corrects, based on the value havingrandomness, the ranking of the selection candidate cell.

Alternatively, the UE 100 (controller) may select, based on the valuehaving randomness, the target cell from the cells operated at afrequency having the same priority as the frequency of the currentserving cell. Here, the frequency having the same priority as thefrequency of the current serving cell may be the same frequency as thefrequency of the current serving cell, or may be a frequency differentfrom the frequency of the current serving cell. In other words, the UE100 (controller) identifies the cells that satisfies the above-describedconditions, and selects, based on the value having randomness, thetarget cell from the identified cells (selection candidate cells). Forexample, the UE 100 (controller) corrects, based on the value havingrandomness, the ranking of the selection candidate cell.

Alternatively, the UE 100 (controller) may select, based on the valuehaving randomness, the target cell from the cells having the quality ina predetermined range. Here, the predetermined range is preferablyincluded in the system information (SIB; System Information Block)broadcast from the current serving cell. In other words, the UE 100(controller) identifies the cells having the quality in thepredetermined range, and may select, based on the value havingrandomness, the target cell from the identified cells (selectioncandidate cells). The quality in the predetermined range may be aquality in which a difference to the best quality is included in thepredetermined range (for example, within 5 dB, and the like), or may bea quality of a cell having a ranking in the predetermined range (forexample, the top three) counted from the top of the ranking. Forexample, the UE 100 (controller) corrects, based on the value havingrandomness, the ranking of the selection candidate cell.

It is noted that, the ranking of the cell can be corrected as follows,for example. If the random number is used as the value havingrandomness, the ranking of the cell is modified by using a function ofRoundup {RAND×(number of selection candidate cells)}. Alternatively, ifthe UE-ID is used as the value having randomness, the cell with thehighest ranking is selected as the target cell from the selectioncandidate cells that satisfy the relationship of (UE ID) mod (Cell ID)n. n is a predetermined value or a value broadcast from the servingcell.

(Mobile Communication Method)

Hereinafter, the mobile communication method according to the firstmodification will be described. FIG. 11 is a flow diagram illustratingthe mobile communication method according to the first modification. Itshould be noted that the flow illustrated in FIG. 11 is performed by theUE 100.

As illustrated in FIG. 11, in step S20, the UE 100 determines whether ornot the start conditions to start measuring the quality of theneighboring cell adjacent to the current serving cell are satisfied. Ifa determination result is YES, the UE 100 performs a process of stepS21. On the other hand, if the determination result is NO, the UE 100terminates the series of processes.

It is noted that, the start conditions are shown as follows, asdescribed above:

(A1) A frequency having a higher priority than the priority of thefrequency of the current serving cell

-   -   the UE 100 always measures the quality of the frequency having        high priority.

(A2) A frequency having a priority equal to or lower than the priorityof the frequency of the current serving cell

-   -   the UE 100 measures, if the quality of the current serving cell        falls below a predetermined threshold value, the quality of the        frequency having equal priority or low priority.

Here, in step 20, as described above, the priority of the frequency(cellReselectionPriority) may be corrected based on the value havingrandomness (for example, the UE-ID, the random number, and the AC).

In step S21, the UE 100 measures the quality of the neighboring celladjacent to the current serving cell.

Here, in step S21, as described above, the quality (for example, Squal,Srxlev, Q_(meas,s), Q_(meas,n)) may be corrected based on the valuehaving randomness (for example, the UE-ID, the random number, and theAC).

In step S22, the UE 100 selects the target cell used as the serving cellfrom the cells (selection candidate cells) having the quality thatsatisfies the predetermined quality criteria (S-criteria or R-criteria).

Here, in step S22, as described above, the ranking of the selectioncandidate cell may be corrected based on the value having randomness(for example, the UE-ID, the random number, and the AC).

It is noted that, in the description according to FIG. 11 a part of thefirst modification is only illustrated, however, it should be noted thatthe UE 100 only needs to select the target cell by using the valuehaving randomness, as described above.

(Operation and Effect)

The UE 100 (controller) according to the first modification selects thetarget cell by using the value having randomness. That is, even for theUEs 100 that are present in the geographically close location, cellsselected as a target cell will vary. Therefore, load distribution ofeach cell can be appropriately performed. In other words, the UE 100 inthe RRC idle state can be appropriately distributed to each cell.

Second Modification

A second modification of the second embodiment will be described, below.Description proceeds with a focus on differences from the secondembodiment, below.

Specifically, in the second embodiment, the UE 100 measures, even if thequality of the current serving cell satisfies the predetermined qualitycriteria, at the predetermined timing, the quality of the neighboringcell adjacent to the current serving cell. On the contrary, in thesecond modification, the UE 100 (controller) selects, as illustrated in(B1) to (B3) described in the second embodiment, the target cell used asthe serving cell, from the cells having the quality that satisfies thepredetermined quality criteria (S-criteria or R-criteria). The UE 100(controller) selects the target cell based on the reselection parameterdifferent for each group including one or more UEs 100.

Here, the group to which the UE 100 belongs is designated by a message(for example, RRC Connection Release) used in a transition procedurefrom the connected state to the idle state. The message (for example,RRC Connection release) may include group identification information foridentifying the group to which the UE 100 belongs.

Further, the reselection parameter includes the priority of thefrequency (cellReselectionPriority), the predetermined period(Treselection_(RAT)), the various types of offsets (Q_(qualminoffset),Q_(rxlevminoffset), Qoffset_(temp), Q_(Hyst), Qoffset), and the varioustypes of threshold values (Thresh_(X,HighQ), Thresh_(X,HighP),Thresh_(Serving,LowQ), Thresh_(Serving,LowP)).

In the second modification, the reselection parameter is preferablyincluded in the system information (SIB; System Information Block)broadcast from the current serving cell. That is, the cell thatbroadcasts the reselection parameter may be different from the cell fortransmitting the message (for example, RRC Connection Release). However,the reselection parameter may be included in the message (for example,RRC Connection Release).

Here, the UE 100 (controller) may maintain the group to which the UE 100belongs until transitioning again from the RRC idle state to the RRCconnected state. In other words, the group to which the UE 100 belongsis released by a transition from the RRC idle state to the RRC connectedstate. Alternatively, the UE 100 (controller) may maintain the group towhich the UE 100 belongs until the timer activated by reception of themessage (for example, RRC Connection Release) expires. In other words,the group to which the UE 100 belongs may be released when the timeractivated by designation of the group expires.

The group including one or more UEs 100 may be formed based on one ormore pieces of information selected from a category of the UE 100 (acapacity of throughput), a capability of the UE 100 (number of streamsof MIMO, and the like), a statistic according to a traffic of the UE 100(S1 Initial UE Context Setup), a statistic according to a mobility ofthe UE 100 (S1 Initial UE Context Setup), and location information ofthe UE 100 (including measurement report).

Alternatively, the group including one or more UEs 100 may be formed bythe access class (AC) related to the access regulation of the UE 100.That is, the reselection parameter differs for each access class relatedto the access regulation of the UE 100.

(Mobile Communication Method)

Hereinafter, the mobile communication method according to the secondmodification will be described. FIG. 12 is a sequence diagramillustrating the mobile communication method according to the secondmodification.

As illustrated in FIG. 12, in step S30, the UE 100 is in the RRCconnected state.

In step S31, the UE 100 receives the message (RRC Connection Release)used in the transition procedure from the connected state to the idlestate. The message (RRC Connection Release) designates the group towhich the UE 100 belongs.

In step S32, the UE 100 is in the RRC idle state.

In step S33, the UE 100 receives the system information (SIB; SystemInformation Block) broadcast from the eNB 200 (current serving cell).The system information includes the reselection parameter different foreach group including one or more UEs 100.

It is noted that, the cell that broadcasts the reselection parameter maybe different from the cell for transmitting the message (for example,RRC Connection Release).

In step S34, the UE 100 selects the target cell based on the reselectionparameter assigned to the group to which the UE 100 belongs.

(Operation and Effect)

The UE 100 (controller) according to the second modification selects thetarget cell based on the reselection parameter different for each groupincluding one or more UEs 100. That is, even for the UEs 100 that arepresent in the geographically close location, cells selected as a targetcell will be different for different groups. Therefore, loaddistribution of each cell can be appropriately performed. In otherwords, the UE 100 in the RRC idle state can be appropriately distributedto each cell.

Third Modification

A third modification of the second embodiment will be described, below.Description proceeds with a focus on differences from the firstmodification, below.

In the first modification, a case where the UE 100 (controller)corrects, based on the value having randomness, the ranking of theselection candidate cell has been illustrated. On the contrary, in thethird modification, another selection method will be explained as aselection method of the target cell based on the value havingrandomness.

Specifically, the UE 100 (receiver) receives the system informationincluding the threshold value to be compared with the random numbergenerated by the controller. The UE 100 (controller) selects the targetcell, based on the comparison result between the random number and thethreshold value.

More particularly, the UE 100 (controller) calculates, based onmeasurement results of the quality of each of the plurality of cells,the rankings of each of the plurality of cells, as described above. Therankings are, as described above, the ranking R_(s) of the currentserving cell and the ranking R_(n) of the neighboring cell. The UE 100(controller) selects the target cell, based on the comparison resultbetween the random number and the threshold value.

In such a case, the system information includes threshold valuesdifferent for each ranking. The UE 100 (controller) generates randomnumbers for each ranking, and selects the target cell, based on thecomparison result between random numbers and threshold values for eachranking. The UE 100 (controller) may compare the random number with thethreshold value in descending order of ranking, and select, as thetarget cell, the cell of which the comparison result between the randomnumber and the threshold value satisfies the selection conditions.

For example, the system information includes information (Ranking_No,Probability) associating the ranking with the threshold value. Suchinformation (Ranking_No, Probability) is information such as {Rank 1,0.6}, {Rank 2, 0.8}, and {Rank 3, 1.0}. It is noted that, {Rank 1, 0.6}means that the threshold value associated with the Rank 1 is 0.6.Similarly, {Rank 2, 0.8} means that the threshold value associated withthe Rank 2 is 0.8, and {Rank 3, 1.0} means that the threshold valueassociated with the Rank 3 is 1.0. Further, the range that the randomnumber generated by the UE 100 (controller) can take is 0 to 1. Here,the selection conditions that the comparison result between randomnumber and the threshold value should satisfy are, for example,conditions that the random number is equal to or less than the thresholdvalue.

Firstly, the UE 100 (controller) determines whether or not to select thecell with ranking 1, as the target cell. That is, the UE 100(controller) generates the random number for the cell with ranking 1,and determines whether or not the comparison result between the randomnumber and 0.6 satisfies the selection conditions. For example, the UE100 (controller) determines that, if the random number is 0.8, theselection conditions are not satisfied. Therefore, the UE 100(controller) continues to select the target cell.

Secondly, the UE 100 (controller) determines whether or not to selectthe cell with ranking 2, as the target cell. That is, the UE 100(controller) generates the random number for the cell with ranking 2,and determines whether or not the comparison result between the randomnumber and 0.8 satisfies the selection conditions. For example, the UE100 (controller) determines, if the random number is 0.7, the selectionconditions are satisfied. Therefore, the UE 100 (controller) selects thecell with ranking 2 as the target cell, and terminates the selection ofthe target cell.

According to the selection method of the above-described target cell,the probability that the cell with ranking 1 is selected as the targetcell is 60%. The probability that the cell with ranking 2 is selected asthe target cell is 32% ({1−0.6}×0.8). The probability that the cell withranking 3 is selected as the target cell is 8% (1−0.6−0.32). Thus, thethreshold value included in the system information may be determined sothat a cell with a high ranking is apt to be selected as a target cell.The third modification is not limited thereto, and the threshold valueincluded in the system information may be determined by the load orcapability of the cell.

In the third modification, a case where the system information includesthe information associating the ranking with the threshold value(Ranking_No, Probability) has been illustrated. However, the thirdmodification is not limited thereto. For example, the system informationincludes the information associating the priority with the thresholdvalue (Priority, Probability).

In such a case, the UE 100 (controller) generates random numbers foreach priority, and selects the target cell based on the comparisonresult between random numbers and threshold values for each priority.The UE 100 (controller) may compare the random number with the thresholdvalue in descending order of priority, and select, as the target cell,the cell of which the comparison result between the random number andthe threshold value satisfies the selection conditions.

The priority may be a priority of the frequency at which the cell isoperated, or a priority unique to the cell.

Fourth Modification

A fourth modification of the second embodiment will be described, below.Description proceeds with a focus on differences from the secondembodiment and the third modification, below.

In the fourth modification, the UE 100 (controller) triggers (orexecutes or starts, the same hereinafter), in response to a referencetrigger notification that is any one of a plurality of triggernotifications broadcast from the current serving cell, an operation (ora procedure, the same hereinafter) for selecting (or reselecting, thesame hereinafter) the target cell. The reference trigger notification ispreferably the trigger notification of any one of the plurality oftrigger notifications. The reference trigger notification may be a partof the trigger notification among the plurality of triggernotifications, or may be two or more trigger notifications.

Here, the trigger notification may be, at the initiative of the network(in this case, the current serving cell), a notification for directly orindirectly triggering the operation in which the UE 100 selects thetarget cell, and a notification broadcast over a plurality of timeswithin a constant period. In the other words, the trigger notificationmay be a notification instructing the UE 100 to perform the operationfor selecting the target cell.

For example, the trigger notification may be the system information(SIB; System Information Block) described in the second embodiment. Thesystem information described in the second embodiment includesinformation for designating the predetermined timing at which the UE 100starts the quality measurement of the neighboring cell. Therefore, itshould be noted that the system information described in the secondembodiment directly triggers the operation in which the UE 100 selectsthe target cell.

Alternatively, the trigger notification may be the system information(SIB; System Information Block) described in the third modification. Asdescribed above, the system information described in the thirdmodification includes the threshold value to be compared with the randomnumber. Alternatively, the system information described in the thirdmodification may include the information associating the ranking withthe threshold value (Ranking_No, Probability). Alternatively, the systeminformation described in the third modification may include theinformation associating the priority with the threshold value (Priority,Probability). It is noted that, the UE 100 (controller) selects, inresponse to reception of the system information described in the thirdmodification, the cell of which the comparison result between the randomnumber and the threshold value satisfies the selection conditions, asthe target cell. Therefore, it should be noted that the systeminformation described in the third modification indirectly triggers theoperation in which the UE 100 selects the target cell.

In such case, each of the plurality of trigger notifications includes acounter value. The UE 100 (controller) stores, in response to trigger(or execution, the same hereinafter) of the operation for selecting thetarget cell, the counter value included in the reference triggernotification in a counter. For example, the UE 100 retains the counterin the memory 150. The UE 100 may accumulate, each time the operationfor selecting the target cell in response to the reference triggernotification is triggered, the counter value in the counter. The UE 100(controller) does not trigger (or prohibits to execute), if the countervalue included in the trigger notification broadcast from the currentserving cell coincides with the counter value stored in the counter, theoperation for selecting the target cell in response to the triggernotification. On the other hand, the UE 100 (controller) triggers, ifthe counter value included in the trigger notification broadcast fromthe current serving cell does not coincide with the counter value storedin the counter, the operation for selecting the target cell. Thus, inresponse to the trigger notification (that is, the reference triggernotification) which is any one of the trigger notifications broadcastover a plurality of times within the constant period, the operation forselecting the target cell by the UE 100 is triggered only once. In otherwords, in response to each of the plurality of trigger notificationswithin the constant period, a situation in which the operation forselecting the target cell is triggered over a plurality of times issuppressed. In other words, the UE 100 triggers, in response to one typeof the trigger notification, the operation for selecting the target cellat most once. It is noted that, the counter value may be a predeterminednumerical value (0 to 9, and the like).

It is noted that, the counter value included in each of the plurality oftrigger notifications is updated by the current serving cell at thetiming when causing the UE 100 to trigger the operation for selectingthe target cell. In other words, the current serving cell broadcasts, atthe timing when attempting to redistribute the UE 100 in the RRC idlestate to each cell, the trigger notification including the updatedcounter value, over a plurality of times within the constant period.

(Operation and Effect)

Here, if the operation for selecting the target cell is triggered over aplurality of times by the UE 100, in response to each of the pluralityof trigger notifications, most of the UEs 100 existing in the currentserving cell result in performing selection of the target cell. In otherwords, most of the UEs 100 result in selecting the similar cell as theserving cell, and as a result, the UEs 100 in the RRC idle state cannotbe appropriately distributed to each cell.

On the contrary, in the fourth modification, the UE 100 (controller)triggers, in response to the reference trigger notification that is anyone of the plurality of trigger notifications broadcast from the currentserving cell, the operation for selecting the target cell. Therefore,the UEs 100 in the RRC idle state can be appropriately distributed toeach cell.

Fifth Modification

A fifth modification of the second embodiment will be described, below.Description proceeds with a focus on differences from the fourthmodification, below.

In the fourth modification, the trigger notification is the systeminformation (SIB; System Information Block) described in the secondembodiment or the third modification. On the contrary, in the fifthmodification, the trigger notification is a reselection request signalfor requesting reselection of the target cell.

In the fifth modification, the UE 100 (receiver) receives, from thecurrent serving cell, the reselection request signal for requestingreselection of the target cell. The UE 100 (controller) performs, inresponse to reception of the reselection request signal, reselection ofthe target cell. Here, it should be noted that the UE 100 (controller)starts, even if the start conditions indicated in the above-described(A1) and (A2) are not satisfied, the quality measurement of theneighboring cell upon reselection of the target cell.

Here, the current serving cell broadcasts, if the load of the currentserving cell is equal to or greater than a predetermined load, thereselection request signal. The current serving cell is preferable torepeatedly broadcast the reselection request signal in a period duringwhich the load of the current serving cell is equal to or greater thanthe predetermined load. In other words, the UE 100 (receiver) receives,if the load of the current serving cell is equal to or greater than thepredetermined load, the reselection request signal from the currentserving cell. The UE 100 (receiver) is preferable to repeatedly receivethe reselection request signal in the period during which the load ofthe current serving cell is equal to or greater than the predeterminedload.

In the fifth modification, the reselection request signal includes areselection parameter for making the current serving cell hard to beselected as the target cell. The UE 100 (controller) selects the targetcell, based on the reselection parameter. More particularly, thereselection parameter may be a parameter that designates to change thepriority of the frequency of the current serving cell(cellReselectionPriority) to the lowest priority, or an offsetindicating the number of steps to lower the priority of the frequency ofthe current serving cell (cellReselectionPriority). Alternatively, thereselection parameter may be the various types of offsets(Q_(qualminoffset), Q_(rxlevminoffset), Qoffset_(temp), Q_(Hyst),Qoffset), and the various types of threshold values (Thresh_(X,HighQ),Thresh_(X,HighP), Thresh_(Serving,LowQ), Thresh_(Serving,LowP)).

In the fifth modification, the reselection parameter applied in responseto reception of the reselection request signal is not included in thereselection request signal, but may be included in the systeminformation (SIB; System Information Block) broadcast separately fromthe reselection request signal, from the current serving cell.Alternatively, the reselection parameter applied in response toreception of the reselection request signal may be predetermined. Insuch a case, the UE 100 (controller) performs, in response to receptionof the reselection request signal, reselection of the target cell, basedon the reselection parameter broadcast from the current serving cell orthe predetermined reselection parameter.

In the fifth modification, the UE 100 (controller) may maintain, if,even if the reselection request signal is received, the predeterminedcondition is satisfied, existence in the current serving cell withoutperforming reselection of the target cell. The predetermined conditionis a condition related to at least one type of information selected froma class of the UE 100 (UE class), the priority of the frequency of thecurrent serving cell, the power consumption setting of the UE 100, thetime elapsed since receiving the last reselection request signal, andthe value having randomness.

For example, if the class of the UE 100 (UE class) is MTC (Machine TypeCommunication) or a data exclusive terminal, a communication data amountis small, and hence, the UE 100 (controller) preferably maintains theexistence in the current serving cell without performing reselection ofthe target cell.

Alternatively, if the priority of the frequency of the current servingcell is the highest priority, there is a high possibility of receivingMBMS data or providing D2D proximity service (D2D ProSe), and hence, theUE 100 (controller) preferably maintains the existence in the currentserving cell without performing the reselection of the target cell.

Alternatively, if the power consumption setting of the UE 100 is the lowpower consumption setting, in order to suppress the power consumption,the UE 100 (controller) preferably maintains the existence in thecurrent serving cell without performing reselection of the target cell.

Alternatively, if the time elapsed since receiving the last reselectionrequest signal has not elapsed the predetermined time, in order tosuppress the ping-pong phenomenon, the UE 100 (controller) preferablymaintains the existence in the current serving cell without performingreselection of the target cell. For example, the UE 100 (controller)activates the timer at the timing of receiving the last reselectionrequest signal, and preferably do not perform reselection of the targetcell until the timer expires.

Alternatively, in order to suppress a situation where the plurality ofUEs 100 perform reselection of the target cell all at once, and randomlydistribute the UEs 100 to each cell, the UE 100 (controller) preferablymaintains the existence in the current serving cell without performingreselection of the target cell, if the value having randomness is otherthan the predetermined value. The value having randomness may be a valuewhich is not common to all UEs 100 that exist in the current servingcell. For example, the value having randomness may be the subframenumber (SFN) in which the UE 100 receives the reselection requestsignal, the identifier (UE-ID) of the UE 100 for receiving thereselection request signal, or the random number generated by the UE100. For example, the UE 100 (controller) performs, if SFNmodUE−ID(÷n)=0 is satisfied, reselection of the target cell, and does notperform, if SFNmodUE−ID (÷n)=0 is not satisfied, reselection of thetarget cell. n is a predetermined value or a value broadcast from theserving cell.

Sixth Modification

A fifth modification of the second embodiment will be described, below.Description proceeds with a focus on differences from any one of thesecond embodiment, the first modification to the fifth modification,below.

Specifically, in the embodiment, the first modification to the fifthmodification, a signal such as the system information (SIB; SystemInformation Block) is broadcast from the current serving cell to the UE100.

On the contrary, in the sixth modification, a signal such as the systeminformation broadcast from the current serving cell is broadcast to theUE 100, and is also notified to the neighboring cell (eNB 200) adjacentto the current serving cell (eNB 200). Alternatively, a load state ofthe current serving cell (eNB 200) is also notified to the neighboringcell (eNB 200) adjacent to the current serving cell (eNB 200). Thesignal or the load state such as the system information are exchanged,via an X2 interface connecting two or more eNBs 200, between each cell.Thus, the signal such as system information broadcast in each cell (theparameter for prompting reselection of the target cell) or the loadstate of each cell are shared between each cell, and hence, cooperativecontrol of the plurality of cells can be performed.

For example, in a case where the loads of the plurality of cellsadjacent to each other are all high, a phenomenon in which reselectionof the target cell frequently occurs between the plurality of cells canbe suppressed (ping-pong phenomenon). More particularly, the currentserving cell puts, in a case where the signal (the parameter forprompting reselection of the target cell) such as the system informationis broadcast in the neighboring cell or a case where the load of theneighboring cell is higher than a threshold value, a broadcast of thesignal (the parameter for prompting reselection of that target cell)such as the system information for the UE 100, on hold.

Here, the signal such as the system information may include, asdescribed in the second embodiment, the parameter for designating atiming (predetermined timing) at which the quality measurement of theneighboring cell is started. Alternatively, the signal such as thesystem information may include, as described in the first modification,the predetermined range referred to in selection of the target cellbased on the value having randomness. Alternatively, the signal such asthe system information may include, as described in the secondmodification, the reselection parameter different for each groupincluding the one or more UEs 100. Alternatively, the signal such as thesystem information may include, as described in the third modification,the threshold value to be compared with the random number generated bythe controller. Alternatively, the signal such as the system informationmay include, as described in the fourth modification, the triggernotification to prompt reselection of the target cell. Alternatively,the signal such as the system information may include, as described inthe fifth modification, the reselection request signal to promptreselection of the target cell, and may include the reselectionparameter applied in response to reception of the reselection requestsignal.

Seventh Modification

A seventh modification of the second embodiment will be described below.Description proceeds with a focus on differences from the fifthmodification, below.

Although not particularly mentioned in the fifth modification, in theseventh modification, the reselection request signal is included in apaging signal broadcast from the eNB 200 (serving cell).

Specifically, a transmitter of the eNB 200 (serving cell) broadcasts thepaging signal on a different paging occasion. The eNB 200 (serving cell)broadcasts the paging signal including the reselection request signalfor requesting reselection of the target cell.

Here, the paging occasion is a subframe including the paging signal. Theone or more paging occasions are included in the radio frame (pagingframe). The paging frame (PF) is calculated byPF=SFNmodT=(TdivN)×(UE_IDmodN). T is a DRX cycle of the UE 100, and N isrepresented by min (T, nB). nB is a value selected from 4T, 2T, T, T/2,T/4, T/8, T/16, and T/32. The paging occasion is defined by therelationship between Ns and i_s. i_s is represented by i_s=floor(UE_ID/N) modNs, and Ns is represented by max (1, nB/T).

In the seventh modification, the eNB 200 (serving cell) broadcasts, asthe paging signal including the reselection request signal, a firstpaging signal on a first paging occasion, and broadcasts, as the pagingsignal not including the reselection request signal, a second pagingsignal on a second paging occasion different from the first pagingoccasion. In other words, the eNB 200 (serving cell) determines, foreach paging cycle, whether or not to include the reselection requestsignal in the paging signal, and broadcasts, for each paging cycle, thepaging signal including the reselection request signal or the pagingsignal not including the reselection request signal.

On the other hand, the UE 100 (receiver) receives the paging signalbroadcast from the current serving cell. It should be noted that the UE100 receives the paging signal at the paging cycle assigned to the UE100. The UE 100 (controller) performs, in response to the reselectionrequest signal included in the paging signal, reselection of the targetcell. It should be noted that the UE 100 starts, similarly to the fifthmodification, even if the start conditions indicated in theabove-described (A1) and (A2) are not satisfied, the quality measurementof the neighboring cell upon reselection of the target cell.

In the seventh modification, the UE 100 preferably performs, even if thepaging signal received in the paging cycle assigned to the UE 100 is notthe paging signal addressed to the UE 100, reselection of the targetcell in response to the reselection request signal included in thepaging signal.

(Operation and Effect)

In the seventh modification, the UE 100 performs, in response to thereselection request signal included in the paging signal, reselection ofthe target cell. In other words, by the trigger on the network (currentserving cell) side, the UE 100 in the RRC idle state is prompted toreselect the target cell. Therefore, load distribution of each cell canbe appropriately performed. In other words, the UE 100 in the RRC idlestate can be appropriately distributed to each cell.

Here, the paging cycle differs for each UE 100, and hence, randomness ofthe UE 100 performing reselection of the target cell in response to thereselection request signal included in the paging signal is secured, andthe UEs 100 in the RRC idle state can be appropriately distributed toeach cell.

In the seventh modification, the eNB 200 (serving cell) determines, foreach paging cycle, whether or not to include the reselection requestsignal in the paging signal, and broadcasts, for each paging cycle, thepaging signal including the reselection request signal or the pagingsignal not including the reselection request signal. Therefore, thepercentage of the UE 100 performing reselection of the target cell inresponse to the reselection request signal included in the paging signalcan be controlled on the eNB 200 (serving cell) side, to some extent.For example, by determining, depending on the load of the eNB 200(serving cell), the percentage of the UE 100 performing reselection ofthe target cell, load distribution of each cell can be appropriatelyperformed.

Eighth Modification

An eighth modification of the second embodiment will be described below.Description proceeds with a focus on differences from the firstmodification, below.

Specifically, in the first modification, a case where the UE 100(controller) corrects, based on the value having randomness (forexample, the UE-ID, the random number, and the AC), the quality (forexample, Squal, Srxlev, Q_(meas,s), Q_(meas,n)) is illustrated. In sucha case, the various types of offsets (Q_(qualminoffset),Q_(rxlevminoffset), Qoffset_(temp), Q_(Hyst), Qoffset) are calculatedby, for example, offset=(default offset)×(UE—ID=n).

On the contrary, in the eighth modification, the UE 100 (controller)corrects, based on the value having randomness (for example, the UE-ID,the random number, and the AC), the various types of threshold values(Thresh_(X,HighQ), Thresh_(X,HighP), Thresh_(Serving,LowQ),Thresh_(Serving,LowP)). The various types of threshold values are, asdescribed above, an example of the various types of parameters used forselecting the target cell. Various types of threshold values arecalculated, for example, by threshold=(default threshold)×(UE-ID÷n). nis a predetermined value or a value broadcast from the serving cell.

Ninth Modification

A ninth modification of the second embodiment will be described below.Differences from the fifth modification will be mainly described below.

In the fifth modification, the UE 100 (controller) performs, in responseto the reception of the reselection request signal, the reselection ofthe target cell. The UE 100 selects the target cell, based on thereselection parameter included in the reselection request signal. Incontrast, in the ninth modification, the UE 100 (controller) performs,in response to a trigger that is without dependence on the reselectionrequest signal, a first reselection process, while performing, inresponse to the reception of the reselection request signal, a secondreselection process different from the first reselection process.

Here, the first reselection process is a process of measuring thequality of a neighboring cell adjacent to the current serving cell ifeither one of the above-described start conditions (A1) to (A2) issatisfied, and selecting the target cell if any one of theabove-described selection conditions (B1) to (B3) is satisfied. Incontrast, the second reselection process is a process to which rulesdifferent from those of the first reselection process are applied in thereselection of the target cell. The rules to be applied to the secondreselection process are as follows.

(Rule 1)

According to rule 1 to be applied to the second reselection process, theUE 100 does not use a priority of a preconfigured frequency(cellReselectionPriority).

For example, in place of the priority of a preconfigured frequency, theUE 100 uses the same priority for the priority of each cell orfrequency. In such a case, only the above-described (A2) may be used asthe start condition. Moreover, only the above-described (B2) may be usedas the selection condition.

Alternatively, the UE 100 may ignore the priority of the preconfiguredfrequency (cellReselectionPriority). In such a case, only either one ofthe above-described (A1) and (A2) may be used as the start condition.Only any one of the above-described (B1) to (B3) may be used as theselection condition.

It is noted that the above-described (B2) is a process of reselectingthe target cell, based on the ranking Rs of the current serving cell andthe ranking Rn of the neighboring cell (ranking process). That is, itshould be noted that in a case in which only (B2) is used, the rankingprocess is performed for all cells in response to the reception of thereselection request signal. Alternatively, it should be noted that theranking process is performed for a cell having a quality equal to ormore than a fixed quality in response to the reception of thereselection request signal.

(Rule 2)

According to rule 2 to be applied to the second reselection process, theUE 100 starts, in response to the reception of the reselection requestsignal, measurement of the quality of the measurement target frequencyincluded in the broadcast information broadcast from the current servingcell. The broadcast information, for example, may be an SIB4 designatingthe frequency to be measured of an Intra-frequency, or may be an SIB5designating the frequency to be measured of an Inter-frequency.

Here, in the first selection process, the UE 100 starts measurement ofthe Intra-frequency if the cell selection quality level (Squal orSrxlev) of the current serving cell is smaller than a threshold value(SIntraSearchP or SIntraSearchQ). Similarly, the UE 100 startsmeasurement of the Inter-frequency if the cell selection quality level(Squal or Srxlev) of the current serving cell is smaller than athreshold value (SInterSearchP or SInterSearchQ). Under such a premise,the UE 100 may ignore the setting of the threshold value (SIntraSearchPor SIntraSearchQ), and start, in response to the reception of thereselection request signal, the measurement of the Intra-frequency.Alternatively, the UE 100 may ignore the threshold value (SInterSearchPor SInterSearchQ), and start, in response to the reception of thereselection request signal, the measurement of the Inter-frequency.Alternatively, the UE 100 may handle the cell selection quality level(Squal or Srxlev) as the minimum value, and start, in response to thereception of the reselection request signal, the measurement of thequality of the measurement target frequency.

(Rule 3)

Rule 3 to be applied to the second reselection process is a rule forselecting the target cell in the ranking process. According to the ruleto be applied to the first process, a cell with the highest ranking, ora cell with a higher ranking than the current serving cell is selectedas the target cell. In contrast, according to rule 3 to be applied tothe second reselection process, the process described below isperformed.

For example, in the selection of the target cell, the UE 100 selects thetarget cell by using a value having randomness. In other words, the UE100 designates a cell having a quality that satisfies the predeterminedquality criteria (S-criteria or R-criteria), and selects, based on thevalue having randomness, the target cell from among the designated cells(candidate selection cells). In such a case, the UE 100 corrects theranking of the candidate selection cells, based on the value havingrandomness. It is noted that the value having randomness is, forexample, a UE-ID, a random number, or an AC.

Alternatively, the UE 100 may select a cell that does not have thehighest ranking as the target cell, or the UE 10 may select a cell thatdoes not have a higher ranking than the current serving cell as thetarget cell.

Alternatively, in the ranking process, the UE 100 may determine theranking of the current serving cell or the neighboring cell, based on aquality (such as the RSRQ or the SINR) other than the RSRP. Such a rulemay be thought of as an extension of the above-described “R-criteria”.

(Rule 4)

According to rule 4 to be applied to the second reselection process, theUE 100 does not use a time threshold value regarding the reselection ofthe target cell. Specifically, the time threshold value is apredetermined period (TreselectionRAT) to be compared with a time whenthe cell selection quality level (Squal or Srxlev) of the neighboringcell continuously satisfies a predetermined condition, or a prohibitiontime threshold value to be compared with a time that has elapsed sinceexistence in the current serving cell (for example, one second). Thatis, in the second reselection process, the UE 100 handles thepredetermined period (TreselectionRAT) or the prohibition time thresholdvalue (for example, one second) as zero. Alternatively, the UE 100ignores the predetermined period (TreselectionRAT) or the prohibitiontime threshold value (for example, one second).

(Rule 5)

According to rule 5 to be applied to the second reselection process, aperiod during which the second reselection process is applied continuesover a constant period.

For example, the second reselection process is applied since the firstcell is selected as the target cell by the second reselection processuntil the second cell is selected as the target cell by the secondreselection process.

Alternatively, the second reselection process is applied in a periodduring which the timer is running. The timer may be activated inresponse to the reception of the reselection request signal, or may beactivated in response to the selection of the target cell in the secondreselection process. The timer activation time may be broadcast from thecurrent serving cell, or may be included in the reselection requestsignal.

Alternatively, the second reselection process may be applied untilreception of a message for canceling the second reselection process (forexample, Reselection Request Cancel).

Tenth Modification

A tenth modification of the second embodiment will be described below.Differences from the second embodiment will be mainly described below.

The second embodiment does not particularly mention about the frequencyto be measured or to be selected (hereinafter called “measurement targetfrequency”), in the reselection process of the cell. The measurementtarget frequency is, generally, included in the broadcast informationbroadcast from the current serving cell. The broadcast information, forexample, may be an SIB4 designating the frequency to be measured of anIntra-frequency, or may be an SIB5 designating the frequency to bemeasured of an Inter-frequency. In contrast, in the tenth modification,a method of narrowing down a measurement target frequency in a processfor appropriately distributing a UE 100 in the RRC idle state to eachcell (hereinafter called “UE distribution process”) will be described.The method of narrowing down a measurement target frequency is, forexample, as follows.

(First Method)

In a first method, a measurement target frequency is narrowed down to afrequency having the same priority as the priority of the frequency(cellReselectionPriority) of the current serving cell.

(Second Method)

In a second method, in addition to an existing priority (firstpriority), an extended priority (second priority) is introduced as thepriority of the frequency. A measurement target frequency is narroweddown to the frequency having the same existing priority as the existingpriority of the frequency of the current serving cell, and having thesame extended priority as the extended priority of the frequency of thecurrent serving cell.

(Third Method)

In a third method, similarly to the second method, in addition to theexisting priority (first priority), the extended priority (secondpriority) is introduced as the priority of the frequency. Themeasurement target frequency is narrowed down to a frequency to whichthe extended priority has been set, without dependence on the existingpriority.

(Fourth Method)

In a fourth method, similarly to the second method, in addition to anexisting priority (first priority), an extended priority (secondpriority) is introduced as the priority of the frequency. Themeasurement target frequency is narrowed down to a frequency having thesame extended priority as the extended priority of the frequency of thecurrent serving cell, without dependence on the existing priority.

(Fifth Method)

In a fifth method, if a frequency having a priority that is higher thanthe priority of the frequency narrowed down by the first method throughthe fourth method is detected, the process of selecting the target cell(that is, the first reselection process described in the ninthmodification) may be applied if any one of the above-described selectionconditions (B1) through (B3) is satisfied.

In the tenth modification, the method of narrowing down the measurementtarget frequency in the UE distribution process is described. Here, theUE distribution process may be any of the methods indicated in thesecond embodiment up to the ninth modification. For example, asdescribed in the first, third, and eighth modifications, the UEdistribution process may be a process of selecting the target cell byusing a value having randomness. Alternatively, as described in thesecond modification, the UE distribution process may be a process ofselecting the target cell by using a different reselection parameter foreach group. As described in the fourth, fifth, seventh, and ninthmodifications, the UE distribution process may be a process of selectingthe target cell in response to a trigger notification (for example, areselection request signal).

Eleventh Modification

An eleventh modification of the second embodiment will be describedbelow. Differences from the fifth modification will be mainly describedbelow.

In the fifth modification, the UE 100 (receiver) receives, from thecurrent serving cell, the reselection request signal for requesting thereselection of the target cell. The UE 100 (controller) performs, inresponse to the reception of the reselection request signal, thereselection of the target cell.

In contrast, in the eleventh modification, if the UE 100 receives thereselection request signal and a predetermined trigger condition issatisfied, the UE 100 may perform the reselection of the target cell.That is, even if the UE 100 (controller) receives the reselectionrequest signal, the UE 100 may not perform the reselection of the targetcell if the predetermined trigger condition is not satisfied.

In the eleventh modification, the predetermined trigger condition is acondition in which the priority of the frequency(cellReselectionPriority) of the current serving cell is not a highpriority.

For example, if the current serving cell is a CSG (Closed SubscriberGroup) cell, the priority of the frequency of the CSG cell is set tohigh priority. In such a case, the predetermined trigger condition isthat the current serving cell is not a CSG cell. Therefore, if the UE100 receives the reselection request signal, and the current servingcell is not a CSG cell, the UE 100 performs reselection of the targetcell. On the other hand, even if the UE 100 receives the reselectionrequest signal, the UE 100 does not perform the reselection of thetarget cell if the current serving cell is a CSG cell. However, if a CSGcell is included in a neighboring cell of the current serving cell, theUE 100 may perform reselection of the target cell even if the currentserving cell is a CSG cell.

Alternatively, if the D2D proximity service (D2D ProSe) is set in thefrequency of the current serving cell, the priority of the currentserving cell is set to high priority. In such a case, the predeterminedtrigger condition is that the D2D proximity service is not set in thefrequency of the current serving cell, or that the D2D proximity servicecan be set even though the UE 100 does not exist in the current servingcell. Therefore, the UE 100 performs reselection of the target cell ifthe UE 100 receives the reselection request signal in a case where theD2D proximity service is not set in the frequency of the current servingcell, or in a case where the D2D proximity service can be set eventhough the UE 100 does not exist in the current serving cell. On theother hand, even if the UE 100 receives the reselection request signal,the UE 100 does not perform reselection of the target cell if the D2Dproximity service is set in the frequency of the current serving cell,and if the D2D proximity service cannot be set unless the UE 100 existsin the current serving cell. It is noted that whether or not the D2Dproximity service can be set is influenced not only by the network (suchas the eNB 200) settings, but also by the capability of the UE 100.

Alternatively, if the UE 100 has an interest in an MBMS service providedin the frequency of the current serving cell, the priority of thecurrent serving cell is set to high priority. In such a case, thepredetermined trigger condition is that the UE 100 does not have aninterest in the MBMS service provided in the frequency of the currentserving cell, or that an MBMS service in which the UE 100 has aninterest is provided in the frequency of a neighboring cell of thecurrent serving cell. Therefore, the UE 100 performs reselection of thetarget cell if the UE 100 receives the reselection request signal in acase where the UE 100 does not have an interest in the MBMS serviceprovided in the frequency of the current serving cell, or in a casewhere the MBMS service in which the UE 100 has an interest is providedin the frequency of the neighboring cell of the current serving cell. Onthe other hand, even if the UE 100 receives the reselection requestsignal, the UE 100 does not perform reselection of the target cell ifthe UE 100 has an interest in the MBMS service provided in the frequencyof the current serving cell, and if the MBMS service in which the UE 100has an interest is not provided in the frequency of the neighboring cellof the current serving cell.

Twelfth Modification

A twelfth modification of the second embodiment will be described below.Differences from the fifth modification will be mainly described below.

In the fifth modification, a case in which the reselection parameter isa parameter for designating to change the priority of the frequency(cellReselectionPriority) of the current serving cell to the lowestpriority is described as an example. In contrast, in the twelfthmodification, the definition of the lowest priority will be clarified.

Specifically, the lowest priority may be a priority that is lower thanthe lowest priority from among priorities that can be set at the network(eNB 200) side.

Alternatively, the lowest priority may be a priority that is lower thanthe lowest priority of priorities (cellReselectionPriorities) includedin the broadcast information (such as the SIB5) broadcast from theneighboring cell of the current serving cell, and included in thebroadcast information received by the UE 100. That is, the lowestpriority may be a priority that is lower than the lowest priority ofpriorities of the neighboring cell.

Here, as already described in the fifth modification, the UE 100 mayreplace the priority (cellReselectionPriority) included in broadcastinformation (an SIB3) broadcast from the current serving cell with thelowest priority.

It is noted that the priority (cellReselectionPriority) can be in therange of 0 to 7, and if a large value represents a high priority, thelowest priority may be a negative value.

In addition, the reselection request signal may include an identifierindicating whether or not to replace the priority of the frequency(cellReselectionPriority) of the current serving cell with the lowestpriority. For example, if the current serving cell provides an MBMSservice, the current serving cell transmits a reselection request signalincluding an identifier indicating that the priority of the frequency ofthe current serving is replaced with the lowest priority. On the otherhand, if the current serving cell does not provide the MBMS service, thecurrent serving cell transmits a reselection request signal including anidentifier indicating that the priority of the frequency of the currentserving cell is not replaced with the lowest priority.

Thirteenth Modification

A thirteenth modification of the second embodiment will be describedbelow. Differences from the fifth modification will be mainly describedbelow.

In the fifth modification, the UE 100 (receiver) receives, from thecurrent serving cell, the reselection request signal for requesting thereselection of the target cell. The UE 100 (controller) performs, inresponse to the reception of the reselection request signal, thereselection of the target cell.

In contrast, in the thirteenth modification, after performingreselection of the target cell in response to the reception of thereselection request signal (hereinafter called “one-shot reselection”),the UE 100 (controller) may further perform reselection of the targetcell (hereinafter called “post-reselection”). Here, as described in thefifth and twelfth modifications, in the one-shot reselection, afterreplacing the priority of the current serving cell (or the frequency ofthe current serving cell) with the lowest priority, the UE 100 selects aneighboring cell (or a neighboring cell with the frequency) having apriority that is higher than the lowest priority as the target cell.

Here, a case in which two or more neighboring cells (or neighboringcells with the frequency) having a priority that is higher than thelowest priority are found in the one-shot reselection is assumed. Insuch a case, by performing the post-reselection after the one-shotreselection, the UE 100 can reselect an optimum cell as the target cell.

The UE 100 may perform the post-reselection only if the priority of theneighboring cell (or the frequency of the neighboring cell) is the sameas the priority of the current serving cell. That is, the UE 100 mayperform the post-reselection if a neighboring cell (or a neighboringcell with the frequency) having the same priority as the priority of thecell (or the frequency of the cell) selected as the target cell in theone-shot reselection exists. Alternatively, the UE 100 may perform thepost-reselection if two or more neighboring cells having the samepriority are found as the target cell in the one-shot reselection. Onthe other hand, the UE 100 may skip the post-reselection if aneighboring cell (or a neighboring cell with the frequency) having thesame priority as the priority of the cell (or the frequency of the cell)selected as the target cell in the one-shot reselection does not exist.Alternatively, the UE 100 may perform the post-reselection if two ormore neighboring cells having the same priority are not found as thetarget cell in the one-shot reselection. Here, the post selectionprocess may be started by implementing the measurement of theneighboring cell (or the frequency of the neighboring cell) having thesame priority.

To put these expressions in different wording, the process described in(A2) in the second embodiment, that is, the process of starting themeasurement of the frequency having a priority that is equal to or lowerthan the priority of the frequency of the current serving cell if thequality of the current serving cell (for example, Srxlev and Squal)falls below a predetermined threshold value (SnonIntraSearchP andSnonIntraSearchQ) is corrected as follows. Specifically, the UE 100starts the measurement of the frequency having a priority that is equalto or lower than the priority of the frequency of the current servingcell if the quality of the current serving cell (for example, Srxlev andSqual) falls below the predetermined threshold value (SnonIntraSearchPand SnonIntraSearchQ), or if the current serving cell is a cell selectedin the one-shot reselection. On the other hand, the UE 100 may not startthe measurement of the frequency having a priority that is equal to orlower than the priority of the frequency of the current serving cell ifthe quality of the current serving cell (for example, Srxlev and Squal)exceeds the predetermined threshold value (SnonIntraSearchP andSnonIntraSearchQ), and if the current serving cell is not a cellselected in the one-shot reselection. That is, the post-reselectionprocess may be skipped in such a case.

It is noted that the priority of the cell (or the frequency of the cell)is included in the broadcast information (such as the SIB3 or the SIB5)broadcast from each cell, and the UE 100 may grasp the priority, basedon the broadcast information.

In the thirteenth modification, only one the post-reselection may beexecuted after the one-shot reselection. That is, two or morepost-reselections may not be performed after the one-shot reselection.

In the thirteenth modification, the UE 100 has a timer that activates inresponse to execution of the one-shot reselection, and the UE 100 mayperform the post-reselection during a period until a timer value reachesa threshold value. The threshold value to be compared with the timervalue may be included in the broadcast information (the SIB or thepaging signal) broadcast from the current serving cell. The thresholdvalue to be compared with the timer value may be included in theabove-described reselection request signal. It is noted that only onethe post-reselection may be executed during the period until the timervalue reaches the threshold value. That is, two or morepost-reselections may not be performed during the period until the timervalue reaches the threshold value. Alternatively, the UE 100 has a timerthat activates in response to the execution of the post-reselection, andthe UE 100 may not execute the next post-reselection during a perioduntil a timer value reaches ae threshold value, and may execute the nextpost reselection if the timer value reaches the threshold value. If thenext post-reselection is executed upon the timer value reaching thethreshold value, the UE 100 may reset or reactivate the timer. However,even during the period until the timer value reaches the thresholdvalue, the UE 100 may execute the quality measurement described in thesecond embodiment above (for example, (A1) through (A2)) and the cellreselection process (for example, (B1) through (B3)). For example, evenduring the period until the timer value reaches the threshold value, theUE 100 may execute quality measurement of the neighboring cell (or thefrequency of the neighboring cell), as well as the cell reselectionprocess if the quality of the current serving cell (for example, Srxlevand Squal) falls below the predetermined threshold value(SnonIntraSearchP and SnonIntraSearchQ).

In the one-shot reselection according to the thirteenth modification,similarly to the fifth modification or the twelfth modification, thepriority (cellReselectionPriority) included in the broadcast information(the SIB3) broadcast from the current serving cell may be replaced withthe lowest priority. In such a case, in the post-reselection, the UE 100may still handle the priority of the cell (or the frequency of the cell)(cellReselectionPriority) for which the one-shot reselection request isperformed as the lowest priority. As a result, even if the network hasan inadequate setting, etc., the ping-pong phenomenon in which the cellfor which the one-shot reselection will be performed is selected as thetarget cell is suppressed.

Fourteenth Modification

A fourteenth modification of the second embodiment will be describedbelow. Differences from the fifth modification will be mainly describedbelow.

In the fourteenth modification, if the eNB 200 (cell) performs a processof prompting the UE 100 to perform reselection of the target cell bytransmission of the reselection request signal (hereinafter called “UEdistribution process”), the eNB 200 may notify an adjacent eNB of amessage including an indication that the UE distribution process is tobe performed, via an X2 interface. The timing of notifying the messageincluding an indication that the UE distribution process is beingperformed may be before performing the UE distribution process, duringthe course of performing the UE distribution process, or afterperforming the UE distribution process. Here, during the course ofperforming the UE distribution process is a period from a timing oftransmitting the first reselection request signal until a timing oftransmitting the last reselection request signal in repeatedlytransmitting the reselection request signal. In such a case, beforeperforming the UE distribution process may be before the timing oftransmitting the first reselection request signal. After performing theUE distribution process may be after the timing of transmitting the lastreselection request signal.

In addition, if the eNB 200 ends the UE distribution process afternotifying the adjacent eNB of the message including an indication thatthe UE distribution process is to be performed, via the X2 interface,the eNB 200 may notify the adjacent eNB of a message including anindication that the UE distribution process has ended, via the X2interface.

Firstly, the adjacent eNB notified that the UE distribution process isto be performed, performs a process of preventing the eNB 200 (cell)that has notified that the UE distribution process is to be performed,from being selected as the target cell (hereinafter called “reselectionrestriction process”).

For example, the adjacent eNB notified that the UE distribution processis to be performed, may set the priority of the eNB 200 (cell) that hasnotified that the UE distribution process is to be performed lower thanthe priority of the cell managed by the adjacent eNB. The adjacent eNBmay broadcast the newly set priority of the eNB 200 (cell).

Alternatively, the adjacent eNB notified that the UE distributionprocess is to be performed, may add the eNB 200 (cell) that has notifiedthat the UE distribution process is to be performed to a list of cellsthat are not to be selected as the target cell (hereinafter called“black list”). The adjacent eNB may broadcast the black list to whichthe eNB 200 (cell) has been added.

Secondly, the adjacent eNB notified that the UE distribution process isto be performed, performs a process of restricting a handover to the eNB200 (cell) that has notified that the UE distribution process is to beperformed (hereinafter called “handover restriction process”).

For example, the adjacent eNB notified that the UE distribution processis to be performed, may exclude the eNB 200 (cell) that has notifiedthat the UE distribution process is to be performed from an eNB 200 tobe measured (Measurement Configuration).

Alternatively, the adjacent eNB notified that the UE distributionprocess is to be performed, may restrict transmission of a handoverrequest to the eNB 200 (cell) that has notified that the UE distributionprocess is to be performed. Alternatively, the adjacent eNB notifiedthat the UE distribution process is to be performed, may restricttransmission of an addition request of an SeNB in Dual Connectivity, tothe eNB 200 (cell) that has notified that the UE distribution process isto be performed. Alternatively, the adjacent eNB notified that the UEdistribution process is to be performed is notified, may transmit amessage about exclusion from an SeNB in Dual Connectivity, to the eNB200 (cell) that has notified that the UE distribution process is to beperformed.

Here, if the adjacent eNB activates in response to start of thereselection restriction process or the handover restriction process, anda timer value reaches a threshold value, the adjacent eNB may cancel thereselection restriction process or the handover restriction process. Thethreshold value to be compared with the timer value may bepreconfigured, or may be included in the message indicating that the UEdistribution process is to be performed, or may be notified from the OAM(Operation, Administration, and Management) server. Alternatively, theadjacent eNB may cancel the reselection restriction process or thehandover restriction process if being notified that the UE distributionprocess has ended.

Fifteenth Modification

A fifteenth modification of the second embodiment will be describedbelow. Differences from the second embodiment will be mainly describedbelow.

In the second embodiment, the value having randomness includes a randomnumber generated by the UE 100 (controller). In contrast, in thefifteenth modification, the value having randomness does not include arandom number generated by the UE 100 (controller). Further, the UE 100determines whether or not to perform reselection of the target cell byusing the value having randomness.

For example, the value having randomness is a value specific to the UE100 (UE-ID). The value specific to the UE 100 (UE-ID) may be anidentifier assigned by the current serving cell (RNTI; Radio NetworkTemporary Identifier), or may be an identifier assigned uniquely to theUE 100 beforehand (IMSI; International Mobile Subscriber Identity), ormay be an identifier assigned depending on location registration of theUE 100 (S-TMSI; SAE-Temporary Mobile Subscriber Identity).

In the fifteenth modification, the UE 100 determines whether or not toperform reselection of the target cell, based on the value specific tothe UE 100 and the value received from the current serving cell.Specifically, the value received from the current serving cell includesa value (Np) defining the probability that reselection of the targetcell is performed and a value (Nr) for securing equity among UEs 100. Npand Nr may be included in the broadcast information broadcast from thecurrent serving cell. In order to secure equity among UEs 100, it ispreferable that Nr changes in a predetermined period. However, Np may bea predetermined value regardless of the broadcast information broadcastfrom the current serving cell. Similarly, Nr may be a predeterminedvalue regardless of the broadcast information broadcast from the currentserving cell.

For example, the UE 100 may perform reselection of the target cell ifthe condition (UE-IDmodNp)=Nr is satisfied. In such a case, the UE 100does not perform reselection of the target cell if the condition(UE-IDmodNp)=Nr is not satisfied. As described above, an RNTI, IMSI, andS-TMSI can be used as the UE-ID. Therefore, in the expression describedabove, the UE-ID may be replaced with any one of the RNTI, the IMSI, andthe S-TMSI.

Here, a timing of determining whether or not to perform reselection ofthe target cell by using a value having randomness may be the“predetermined timing” described in the second embodiment. Thepredetermined timing is designated, for example, based on the systeminformation (SIB; System Information Block) received from the currentserving cell. It is noted that the predetermined timing means a timingat which the quality measurement of the neighboring cell is startedaccompanied with the selection of the target cell.

As described in the second embodiment, the system information may meanthat the quality measurement of the neighboring cell is startedimmediately. In such a case, the UE 100 designates the timing at whichthe system information is received as the predetermined timing, andstarts the quality measurement of the neighboring cell in response tothe reception of the system information.

Alternatively, the system information may include a subframe number atwhich the quality measurement of the neighboring cell should be started.The UE 100 designates the subframe number included in the systeminformation as the predetermined timing, and starts the qualitymeasurement of the neighboring cell at the designated subframe number.

Alternatively, the system information may include information indicatinga period of the predetermined timing. For example, the systeminformation includes a timer value to be set to a timer activated at atiming when the quality measurement of the neighboring cell, thereselection of the target cell, or the re-distribution process ends. TheUE 100 designates a timing at which the timer to which the timer valueis set expires, as the predetermined timing, and starts the qualitymeasurement of the neighboring cell at the designated timing. It isnoted that the re-distribution process includes an OSS (One Shot Scheme)or a CRS (Continuous Re-Distribution Scheme) described later.

In the fifteenth embodiment, a case in which the value (Nr) for securingequity among UEs 100 is included in the broadcast information broadcastfrom the current serving cell has been described as an example. However,the fifteenth modification is not limited thereto. For example, a framenumber or subframe number including a predetermined timing may be usedas Nr. For example, if the system information means that the qualitymeasurement of the neighboring cell is started immediately, the framenumber or subframe number at which the system information is received isused as Nr. Alternatively, if the system information includes thesubframe number at which the quality measurement of the neighboring cellis to be started, the frame number or the subframe number at which thequality measurement of the neighboring cell is to be started is used asNr.

Alternatively, a value (systemInfoValueTag) incremented in response tothe update of the contents of the system information (SIB) may be usedas the value (Nr) for securing equity among UEs 100. ThesystemInfoValueTag is included in the broadcast information (SIB1)broadcast from the current serving cell.

It is noted that the value of Nr is preferably selected such that therelationship Nr<Np is satisfied. Alternatively, the UE 100 may performreselection of the target cell if the condition (UE-IDmodNp)=(NrmodNp)is satisfied. In such a case, the UE 100 does not perform reselection ofthe target cell if the condition (UE-IDmodNp)=(NrmodNp) is notsatisfied.

Here, the UE 100 may perform reselection of the target cell if one ormore conditions selected from among the three types of conditions of(UE-IDmodNp)>(NrmodNp), (UE-IDmodNp)<(NrmodNp), and(UE-IDmodNp)=(NrmodNp) are satisfied. The one or more conditionsselected from among the three types of conditions (that is, an equalitysign or an inequality sign in the expression described above) may beincluded in the broadcast information broadcast from the current servingcell.

Alternatively, a plurality of Nrs (for example, Nr1 and Nr2) may be setas the value (Nr) for securing equity among UEs 100. In such a case, theUE 100 may perform reselection of the target cell if the condition(UE-IDmodNp)=(Nr1modNp), or the condition (UE-IDmodNp)=(Nr2modNp) issatisfied.

Alternatively, a plurality of Nps (for example, Np1 and Np2) may be setas the value (Np) for defining the probability that reselection of thetarget cell is performed. In such a case, the UE 100 may performreselection of the target cell if the condition(UE-IDmodNp1)=(Nr1modNp1), or the condition (UE-IDmodNp2)=(Nr2modNp2) issatisfied.

Sixteenth Modification

A sixteenth modification of the second embodiment will be describedbelow. Differences from the second embodiment will be mainly describedbelow.

In the sixteenth embodiment, if a re-distribution parameter fordistributing the target cell selected as the serving cell is broadcastfrom the current serving cell, the UE 100 (receiver) receives there-distribution parameter.

The re-distribution parameter includes one or more parameters selectedfrom among the priority of the frequency (cellReselectionPriority), thepriority of the cell (CSP; Cell Specific Priority), a predeterminedperiod (TreselectionRAT), various types of offset (Qqualminoffset,Qrxlevminoffset, Qoffsettemp, QHyst, Qoffset), and various types ofthreshold value (ThreshX, HighQ, ThreshX, HighP, ThreshServing, LowQ,ThreshServing, LowP). The re-distribution parameter is, for example,included in the system information (SIB; System Information Block)broadcast from the current serving cell.

Under such a premise, if the UE 100 (controller) receives there-distribution parameter from the current serving cell, the UE 100performs, in response to the reception of the reselection requestsignal, a first re-distribution process using the re-distributionparameter. On the other hand, if the UE 100 (controller) does notreceive the re-distribution parameter from the current serving cell, theUE 100 performs, in response to the reception of the reselection requestsignal, a second re-distribution process not using the re-distributionparameter. It is noted that similarly to the seventh modification, thereselection request signal may be included in a paging signal broadcastfrom the eNB 200 (serving cell).

Specifically, in the first re-distribution process, the UE 100 performsthe process similar to that in the fifth modification, that is, the UE100 performs reselection of the target cell in response to the receptionof the reselection request signal. However, as described above, there-distribution parameter is used in the first re-distribution process.

On the other hand, in the second re-distribution process, the UE 100makes the current serving cell hard to be selected as the target cell.For example, the following process can be assumed as the secondre-distribution process.

Firstly, in the second re-distribution process, the UE 100 mayrelatively lower the priority of the frequency (cellReselectionPriority)of the current serving cell. For example, the UE 100 changes thepriority of the frequency (cellReselectionPriority) of the currentserving cell to the lowest priority. Alternatively, the UE 100 mayincrease the priority of the frequency of a cell other than the currentserving cell.

Secondly, in the second re-distribution process, the UE 100 mayrelatively lower the priority (CSP) of the current serving cell. Forexample, the UE 100 changes the priority (CSP) of the current servingcell to the lowest priority. Alternatively, the UE 100 may increase thepriority of a cell other than the current serving cell.

Thirdly, in the second re-distribution process, if the priority (CSP) ofthe cell has been set to at least one of cells having the same frequencyas the frequency of the current serving cell, the UE 100 may relativelylower the priority (CSP) of the current serving cell. On the other hand,in the second re-distribution process, if the priority (CSP) has notbeen set to all cells having the same frequency as the frequency of thecurrent serving cell, the UE 100 may relatively lower the priority ofthe frequency (cellReselectionPriority) of the current serving cell.

Fourthly, the reselection request signal may include a predeterminedvalue for designating the priority to be applied in the secondreselection process. In such a case, if the predetermined value is afirst value (for example, 0), the UE 100 may relatively lower thepriority of the frequency of the current serving cell in the secondre-distribution process, and if the predetermined value is a secondvalue (for example, 1), the UE 100 may relatively lower the priority(CSP) of the current serving cell in the second re-distribution process.

Seventeenth Modification

A seventeenth modification of the second embodiment will be describedbelow. Differences from the second embodiment will be mainly describedbelow.

In the seventeenth modification, the UE 100 (receiver) receives, fromthe current serving cell, an indicator (OSS applicable) for indicatingwhether or not to apply the re-distribution process (OSS (One ShotScheme)) of performing reselection of the target cell, in response tothe reception of the reselection request signal. The indicator is, forexample, included in the system information (SIB; System InformationBlock) broadcast from the current serving cell.

Under such a premise, if the UE 100 (controller) receives an indicatorindicating application of the re-distribution process (OSS), and doesnot receive the re-distribution parameter from the current serving cell,the UE 100 may perform, in response to the reception of the reselectionrequest signal, the second re-distribution process. If the UE 100(controller) receives an indicator indicating non-application of there-distribution process (OSS), and does not receive the re-distributionparameter from the current serving cell, the UE 100 may perform, inresponse to the reception of the reselection request signal, the secondre-distribution process.

It is noted that if the UE 100 receives the indicator indicatingnon-application of the re-distribution process (OSS), and receives there-distribution parameter from the current serving cell, the UE 100performs, in response to a trigger without dependence on the reselectionrequest signal, the reselection of the target cell by using there-distribution parameter (CRS; Continuous Re-Distribution Scheme). Thetrigger without dependence on the reselection request signal is, forexample, a trigger such as expiry of a timer. The value set to the timeris, for example, included in system information (SIB; System InformationBlock) broadcast from the current serving cell.

Eighteenth Modification

An eighteenth modification of the second embodiment will be describedbelow. Differences from the second embodiment will be mainly describedbelow.

In the eighteenth modification, the UE 100 (receiver) receives abroadcast existence indicator indicating whether or not there-distribution parameter is being broadcast. The broadcast existenceindicator is, for example, included in system information (SIB; SystemInformation Block) broadcast from the current serving cell. However, thebroadcast existence indicator is preferably included in the systeminformation (for example, an MIB or an SIB1) that is to be acquiredbefore receiving the system information (for example, the SIB3)including the re-distribution parameter. Alternatively, the broadcastexistence indicator may be included in the paging signal including thereselection request signal.

Further, the UE 100 (receiver) receives a re-acquisition indicatorindicating whether or not re-acquisition of the re-distributionparameter is necessary. If the re-acquisition indicator indicates thatthe re-acquisition of the re-distribution parameter is necessary, the UE100 re-acquires the re-distribution parameter. The re-acquisitionindicator may be included in the system information (SIB; SystemInformation Block) broadcast from the current serving cell, or may beincluded in the paging signal including the reselection request signal.

In such a case, if the re-acquisition indicator indicates that there-acquisition of the re-distribution parameter is necessary, the UE 100(controller) performs the first re-distribution process by using there-distribution parameter after the re-acquisition. That is, even if theUE 100 receives the reselection request signal, the UE 100 keeps thefirst re-distribution process pending until the re-distributionparameter is re-acquired, and performs the first re-distribution processafter the re-distribution parameter is re-acquired. On the other hand,if the re-acquisition indicator indicates that the re-acquisition of there-distribution parameter is not necessary, the UE 100 (controller)performs the first re-distribution process by using the re-distributionparameter prior to re-acquisition. That is, if the UE 100 receives thereselection request signal, the UE 100 performs the firstre-distribution process without waiting for the re-acquisition of there-distribution parameter.

In the eighteenth modification, the broadcast existence indicator may beinformation indicating whether or not the re-distribution parameter isbroadcast in the next system modification period (SI ModificationPeriod). The re-acquisition indicator may be information indicatingwhether or not the re-distribution parameter is modified in the nextsystem modification period (SI Modification Period). According to such aconfiguration, the broadcast existence indicator and the re-acquisitionindicator can be notified only to a certain UE 100.

In addition, in a case in which both the re-distribution parameteraccording to the priority of the frequency, and the re-distributionparameter according to the priority of the cell are broadcast from thecurrent serving cell to a single frequency, an application parameterindicator indicating which re-distribution parameter is to be appliedmay be broadcast from the current serving cell. For example, theapplication parameter indicator indicates whether to apply there-distribution parameter according to the priority of the frequency, orto apply the re-distribution parameter according to the priority of thecell, or else to apply the re-distribution parameter according to boththe priority of the frequency and the priority of the cell. Theapplication parameter indicator may be included in the systeminformation (SIB; System Information Block) broadcast from the currentserving cell, or may be included in the paging signal including thereselection request signal. The UE 100 that receives the applicationparameter indicator determines the re-distribution parameter to be usedin the re-distribution process, based on the application parameterindicator.

Nineteenth Modification

A nineteenth modification of the second embodiment will be describedbelow. Differences from the second embodiment will be mainly describedbelow.

In the nineteenth modification, similarly to in the fifteenthmodification, the UE 100 (controller) performs reselection of the targetcell, based on a value specific to the UE 100. In particular, inaddition to the value specific to the UE 100, the UE 100 uses valuesthat are different for each frequency or cell to perform the firstre-distribution process or the second re-distribution process.

For example, the UE 100 performs the first re-distribution process orthe second re-distribution process according to the followingexpression.

$\begin{matrix}{\left\lbrack {{Math}.\mspace{14mu} 1} \right\rbrack \mspace{520mu}} & \; \\{{{redistrRange}\lbrack i\rbrack} = {{{redistrFactor}\lbrack i\rbrack}/{\sum\limits_{j = 0}^{j = {({{\max \; {CandidateFreq}} - 1})}}{{redistrFactor}\lbrack j\rbrack}}}} & {{expression}\mspace{14mu} (1)} \\{\left\{ {\left( {{UE\_ ID} + {offset}} \right){mod}\; 100} \right\}<={100 \times {{redistrRange}\lbrack 0\rbrack}}} & {{expression}\mspace{14mu} (2)} \\{{100 \times {\sum\limits_{j = 0}^{j = {i - 1}}{{redistrRange}\lbrack j\rbrack}}} < \left\{ {\left( {{UE\_ ID} + {offset}} \right){mod}\; 100} \right\}<={100 \times {\sum\limits_{j = 0}^{j = i}{{redistrRange}\lbrack j\rbrack}}}} & {{expression}\mspace{14mu} (3)}\end{matrix}$

However, redistrRange[i] is a value for defining the probability that acell with the ith candidate frequency is selected as the target cell,and is a value being from 0 to 1. redistrRange[0] is a value fordefining the probability that a cell with the frequency of the currentserving cell is selected as the target cell, and is a value being from 0to 1. UE_ID is a value specific to a UE, offset is a value that isdifferent for each frequency or cell, and is a value being from 0 to100.

In such a case, if offset is not used, a certain UE 100 that satisfies“(UE_IDmod100)≦100×redeitrRange[0]” does not move from the currentserving cell to a cell with another frequency, and continues to exist inthe serving cell. That is, the certain UE 100 that need not move fromthe current serving cell is fixed. If such a situation is permitted, aninequity may occur between the certain UE 100 and the other UEs 100. Incontrast, in the nineteenth modification, the inequity among the UEs 100can be resolved by the introduction of the offset.

Further, if the offset is not used, the effect of re-distribution of theUE 100 to each cell is influenced by the initial distribution of thecertain UE 100 satisfying “(UE_IDmod100)≦100×redeitrRange[0]”. Incontrast, in contrast, in the nineteenth modification, the certain UE100 satisfying “{(UE_ID+offset) mod100}≦100×redeitrRange[0]” can beintentionally induced in a certain frequency by the introduction of theoffset. In addition, if the randomness of UE_ID of the UE 100 thatexists in each cell is taken into consideration, the UE 100 can beeffectively redistributed to each cell as compared to the case in whichthe effect of re-distribution of the UE 100 to each cell is influencedby the initial distribution of the certain UE 100 satisfying“(UE_IDmod100)≦100×redeitrRange[0]”.

In the nineteenth modification, the offset may be, for example, includedin the system information (SIB; System Information Block) broadcast fromthe current serving cell, or may be included in the paging signalincluding the reselection request signal. The offset may be included inthe RRC message transmitted individually to the UE 100.

In the nineteenth modification, the offset may be calculated, based on avalue and another parameter that vary for each frequency or each cell.For the other parameter, time information (SFN or UTC) including atiming when the UE 100 receives the reselection request signal can beused. The offset may be a value that varies for each frequency or cell,or may be a value that is set by OAM.

In the nineteenth modification, a case in which after the expression (2)is applied, the expression (3) is applied is assumed, and in order toselect a UE 100 that moves to the cell with the ith candidate frequency,the order of substitution of (UE_ID+offset) mod100) in the expression(3) is optional. However, the nineteenth modification is not limitedthereto.

Specifically, in a case in which the frequencies assigned toredeitrRange[0] through redeitrRange[j] are predetermined, the order ofsubstitution of (UE_ID+offset) mod100} in expression (2) or expression(3) may be specified. Alternatively, in a case in which (UE_ID+offset)mod100} is substituted in expression (2) or expression (3) in the orderof 0 to j, the frequencies assigned to redeitrRange[0] throughredeitrRange[j] may be specified. The re-distribution process can beintentionally controlled so that the UE 100 can use a cell with acertain frequency as the serving cell by such an order specification orfrequency specification. The information including the above-describedorder specification or frequency specification may be included in thesystem information (SIB; System Information Block) broadcast from thecurrent serving cell, or may be included in the paging signal includingthe reselection request signal. Without the order specification orfrequency specification, the process of substituting (UE_ID+offset)mod100} in expression (2) or expression (3) of redeitrRangecorresponding to an absent order or frequency may be skipped.

Twentieth Modification

A twentieth modification of the embodiment will be described below.Differences from the fifth modification will be mainly described below.

An application scene of the twentieth modification is as illustrated inFIG. 13. As illustrated in FIG. 13, the mobile communication system hasan area #1 and an area #2. A cell #1-1 with a frequency F1 and a cell#1-2 with the frequency F1 are provided in the area #1. A cell #2-1 withthe frequency F1, a cell #2-2 with the frequency F1, and a cell #2-3with the frequency F1 are provided in the area #2. The cells (#1-1 and#2-1) with the frequency F1 are cells having high load, and the cells(#1-2 and #2-2) with a frequency F2 are cells having low load. The cell(#2-3) with a frequency F3 is a cell having medium load, and has ahigher priority than the cells with the frequencies F1 and F2. Forexample, the cells with the frequencies F1 and F2 may be macro cells,and the cell with frequency F3 may be a CSG cell.

In the twentieth modification, similarly to in the fifth modification,the UE 100 (controller) performs, in response to the reception of thereselection request signal, the reselection of the target cell for thecell with the target frequency, while activating a predetermined timer(T360). The UE 100 (controller) uses the cell with the target frequencyas the target cell while the predetermined timer is running. Forexample, in the case illustrated in FIG. 13, if a reselection requestsignal is received from the cell #1-1 with the frequency F1 in the firstarea, reselection of the target cell is performed from the cell #1-1with the frequency F1 to the cell #1-2 with the frequency F2 (anoperation A illustrated in FIG. 13). After such an operation A, a casein which the UE 100 moves from the area #1 to the area #2 is assumed (anoperation B illustrated in FIG. 13).

In such a case, in the twentieth modification, the UE 100 (controller)changes an expiry time of the predetermined timer (T360), based on themovement speed of the UE 100. Specifically, the faster the movementspeed of the UE 100, the UE 100 (controller) sets a short expiry timefor the expiry time of the predetermined timer. For example, if a counttime of the predetermined timer reaches a predetermined time thresholdvalue (T-re-distribution), the UE 100 (controller) determines that thepredetermined timer has expired. Therefore, the predetermined timethreshold value (T-re-distribution) is a value that defines the expirytime of the predetermined timer. A method of changing the expiry time ofthe predetermined timer is, for example, as follows.

(1) The UE 100 (controller) may determine the expiry of the timer byusing a value obtained by multiplying a scaling factor based on themovement speed of the UE 100 with the predetermined time threshold value(T-re-distribution) that defines the expiry time of the predeterminedtimer. The scaling factor is a value that is defined based on themovement speed, and is, for example, a value being from 0 to 1. Thefaster the movement speed, the smaller the value of the scaling factor.

For example, a case is assumed in which a movement speed slower than apredetermined movement speed is considered as a low movement speed, anda movement speed faster than the predetermined movement speed isconsidered as a high movement speed. If the scaling factor correspondingto the low movement speed is 1, the scaling factor corresponding to thehigh movement speed is, for example, 0.25. Therefore, upon the movementspeed of the UE 100 being a low movement speed, if the count time of thepredetermined timer reaches a value obtained by multiplying the scalingfactor (1) with the predetermined time threshold value(T-re-distribution), it is determined that the predetermined timer hasexpired. On the other hand, upon the movement speed of the UE 100 beinga high movement speed, if the count time of the predetermined timerreaches a value obtained by multiplying the scaling factor (0.25) withthe predetermined time threshold value (T-re-distribution), it isdetermined that the predetermined timer has expired.

(2) The UE 100 (receiver) receives a plurality of predetermined timethreshold values that define the expiry time of the predetermined timer.The UE 100 (controller) determines the expiry of the timer by using thepredetermined time threshold value selected based on the movement speedof the UE 100 from among a plurality of predetermined time thresholdvalues. The faster the movement speed, the shorter the predeterminedtime threshold value.

For example, a case is assumed in which a movement speed slower than apredetermined movement speed is considered as a low movement speed, anda movement speed faster than the predetermined movement speed isconsidered as a high movement speed. The predetermined time thresholdvalue corresponding to the high movement speed (T-re-distribution_2) isshorter than the predetermined time threshold value corresponding to thelow movement speed (T-re-distribution_1). The plurality of predeterminedtime threshold values may be included in the system information (SIB)broadcast from an eNB 200 having a high priority cell (#2-3) with ahigher priority than the priority of the cells (#1-2 and #2-2) with thetarget frequency F2.

(3) The UE 100 (controller) may determine the expiry of the timer, basedon the combination of (1) and (2). That is, the UE 100 (controller) maydetermine the expiry of the timer by using a value obtained bymultiplying a scaling factor with the predetermined time threshold valueselected based on the movement speed of the UE 100.

In any one of the above-described procedures (1) through (3), the UE 100(controller) may change the expiry time of the predetermined timer ifthe movement speed of the UE 100 changes after activating thepredetermined timer. It is natural for the UE 100 to change the expirytime of the predetermined timer, based on the movement speed after thechange.

In such a case, if the movement speed of the UE 100 becomes faster thanthe movement speed of the UE 100 that is referenced in activating thepredetermined timer, the UE 100 (controller) may change the expiry timeof the predetermined timer. If the movement speed of the UE 100 becomesslower than the movement speed of the UE 100 that is referenced inactivating the predetermined timer, the UE 100 (controller) may notchange the expiry time of the predetermined timer.

The UE 100 (controller) may change the expiry time of the predeterminedtimer while the predetermined timer is running. The UE 100 (controller)may change the expiry time of the predetermined timer after the expiryof the predetermined timer.

According to the twentieth modification, while the predetermined timeris running, if the possibility is high that the UE 100 moves from thearea #1 in which the UE 100 is located when the UE 100 receives thereselection request signal, to the area #2 in which a high priority cell(#2-3) having a higher priority than the cells (#1-2 and #2-2) with thetarget frequency F2 is provided, it is possible to suppress thesituation where the high priority cell is not selected as the targetcell until the predetermined timer expires. On the other hand, if thepossibility is low that the UE 100 moves from the area #1 to the area#2, while the predetermined timer is running, it is possible to suppressthe ping-pong phenomenon that occurs between a source frequency and atarget frequency.

Twenty First Modification

A twenty first modification of the embodiment will be described below.Differences from the twentieth modification will be mainly describedbelow. An application scene of the twenty first modification is similarto that of the twentieth modification.

In the twentieth modification, the UE 100 (controller) changes an expirytime of the predetermined timer (T360), based on the movement speed ofthe UE 100. In contrast, in the twenty first modification, the UE 100(controller) stops the predetermined timer upon reception of a stoprequest signal for requesting stop of the predetermined timer. The stoprequest signal may be broadcast from an eNB 200 having a high prioritycell (#2-3) with a higher priority than the priority of the cells (#1-2and #2-2) with the target frequency F2. That is, the stop request signalmay not be broadcast from the eNB 200 having a cell provided in the area#1. The stop request signal may be included in the system information(SIB) broadcast from the eNB 200 having a cell provided in the area #2.

Here, the UE 100 (controller) may stop the predetermined timer if themovement speed of the UE 100 is faster than a predetermined speed, andthe UE 100 receives the stop request signal. That is, even if the UE 100receives the stop request signal, the UE 100 may not stop thepredetermined timer if the movement speed of the UE 100 is slower thanthe predetermined speed. However, regardless of the movement speed ofthe UE 100, the UE 100 (controller) may stop the predetermined timer ifthe UE 100 receives the stop request signal.

According to the twenty first modification, similarly to in thetwentieth modification, if the possibility that the UE 100 moves fromthe area #1 to the area #2 is high while the predetermined timer isrunning, it is possible to suppress the situation where the highpriority cell is not selected as the target cell until the predeterminedtimer expires. On the other hand, if the possibility is low that the UE100 moves from the area #1 to the area #2, while the predetermined timeris running, it is possible to suppress the ping-pong phenomenon thatoccurs between a source frequency and a target frequency.

Twenty Second Modification

A twenty second modification of the embodiment will be described below.Differences from the twentieth modification will be mainly describedbelow. An application scene of the twenty second modification is similarto that of the twentieth modification.

In the twentieth modification, the UE 100 (controller) changes an expirytime of the predetermined timer (T360), based on the movement speed ofthe UE 100. In contrast, in the twenty second modification, the UE 100(controller) performs, in response to reception of an inter-frequencyreselection request signal while the predetermined timer is running, thereselection of the target cell for a cell with an inter-frequency. Theinter-frequency reselection request signal may be broadcast from an eNB200 having a high priority cell (#2-3) with a higher priority than thepriority of the cells (#1-2 and #2-2) with the target frequency F2. Thatis, the inter-frequency reselection request signal may not be broadcastfrom the eNB 200 having a cell provided in the area #1. Theinter-frequency reselection request signal may be included in the systeminformation (SIB) broadcast from the eNB 200 having a cell provided inthe area #2.

In the twenty second modification, in response to the reception of theinter-frequency reselection request signal, the predetermined timer maynot be stopped even if reselection of the target cell for a cell with aninter-frequency is performed.

Here, the UE 100 (controller) may perform reselection of the target cellfor a cell with an inter-frequency, based on a parameter broadcast fromthe cell (cell #2-2) with the target frequency (F2), rather than aparameter broadcast from the cell (cell #1-1) with the source frequency(F1). Various types of parameter broadcast from the cell (cell #2-2)with the target frequency (F2) include, for example, the priority of thefrequency (cellReselectionPriority), a predetermined period(TreselectionRAT), various types of offset (Qqualminoffset,Qrxlevminoffset, Qoffsettemp, QHyst, Qoffset), and various types ofthreshold value (ThreshX, HighQ, ThreshX, HighP, ThreshServing, LowQ,ThreshServing, LowP).

According to the twenty second modification, similarly to in thetwentieth modification, if the possibility that the UE 100 moves fromthe area #1 to the area #2 is high while the predetermined timer isrunning, it is possible to suppress the situation where the highpriority cell is not selected as the target cell until the predeterminedtimer expires. On the other hand, if the possibility is low that the UE100 moves from the area #1 to the area #2, while the predetermined timeris running, it is possible to suppress the ping-pong phenomenon thatoccurs between a source frequency and a target frequency.

Other Modifications of Second Embodiment

Although not particularly mentioned in the second embodiment, thequality measurement of the neighboring cell upon selection of the targetcell may be started if the predetermined condition is satisfied at thepredetermined timing (second embodiment). Alternatively, selection ofthe target cell by using the value having randomness (firstmodification) may be performed when the predetermined condition issatisfied. For example, the predetermined condition may be that theaccess class of the UE 100 is the access class notified from the currentserving cell. Alternatively, the predetermined condition may be that thepriority of the frequency modAC=0 is satisfied, or that cell−IDmodAC=0is satisfied.

As described in the second embodiment, a selection procedure of thetarget cell as a whole includes, (A) if the start conditions aresatisfied, a procedure for measuring the quality of the neighboring celladjacent to the current serving cell (a measurement procedure), and (B)a procedure for selecting, from the cells that satisfy the selectionconditions, the target cell used as the serving cell (the selectionprocedure). The second embodiment relates to the measurement procedure,and the first modification to the third modification relate to theselection procedure. In such a case, the measurement procedure accordingto the second embodiment may be combined with the selection procedureaccording to any one of the first modification to the thirdmodification. Further, the selection procedures according to two or moremodifications selected from the first modification to the thirdmodification may be combined.

In the fourth modification, a case where the current serving cell uses,in order to cause the UE 100 to trigger the operation for selecting thetarget cell in response to the reference trigger notification which isany one of the plurality of trigger notifications, the counter that theUE 100 has and the counter value included in the trigger notificationhas been illustrated. However, the fourth modification is not limitedthereto. Specifically, in response to the trigger notification(reference trigger notification) which is any one of the triggernotifications broadcast from the current serving cell, over theplurality of times within the constant period, the operation in whichthe UE 100 selects the target cell may be triggered. Therefore, the UE100 may activate the timer in response to trigger (or execution orstart, the same hereinafter) of the operation for selecting the targetcell, and be configured, even if the trigger notification is receivedduring the time until timer expires, not to trigger the operation forselecting the target cell. The time until the timer expires since thetimer has been activated is preferably comparable to the period (theabove-described constant period) during which the trigger notificationis broadcast over a plurality of times. Alternatively, each of theplurality of trigger notifications include toggle information that cantake a value of two or more, and the UE 100 (controller) may store, ifthe operation for selecting the target cell is triggered in response tothe reference notification, the toggle information included in thereference trigger, in a storage area. In such a case, the UE 100(controller) does not trigger, if the toggle information included in thetrigger notification broadcast from the current serving cell coincideswith the toggle information stored in the storage area, the operationfor selecting the target cell. On the other hand, the UE 100(controller) triggers, if the toggle information included in the triggernotification broadcast from the current serving cell does not coincidewith the toggle information stored in the storage area, the operationfor selecting the target cell.

Although not particularly mentioned in the second embodiment, a cell(hereinafter, an overload cell) that transmits the trigger notificationsuch as a parameter for prompting reselection of the target cell or thereselection request signal for requesting reselection of the target cellmay be handled as follows. Specifically, the UE 100 may exclude theoverload cell from the measurement target cells. Alternatively, the UE100 may exclude, even if the overload cell satisfies the criteria(“S-criteria”) to be satisfied by the neighboring cell, the overloadcell from the target cells used as the serving cell. Alternatively, theUE 100 may exclude the overload cell from the cell imparting theranking. Thus, in a state where the plurality of neighboring cells aretransmitting the trigger notification (for example, in a state where theload is high in all the plurality of neighboring cells), the ping-pongphenomenon in which the UE 100 alternately selects these neighboringcells as the target cell is suppressed.

Although not particularly mentioned in the second embodiment, the UE 100may perform, if the condition that the time elapsed since the UE 100existed in the current serving cell exceeds a prohibition time thresholdvalue (for example, one second) is satisfied, reselection of the targetcell. Under such premises, if the UE 100 performs reselection of thetarget cell in response to the above-described trigger notification, itis preferable to use the time threshold value longer than the timethreshold value used in a case of performing reselection of the targetcell in response to the above-described start conditions (A1 and A2).Thus, in a state where the plurality of neighboring cells aretransmitting the trigger notification (for example, in a state where theload is high in all the plurality of neighboring cells), the ping-pongphenomenon in which the UE 100 alternately selects these neighboringcells as the target cell is suppressed. From a similar point of view, ifthe UE 100 performs reselection of the target cell in response to theabove-described trigger notification, it is preferable to use apredetermined period (Treselection_(RAT)) longer than the predeterminedperiod (Treselection_(RAT)) used in a case of performing reselection ofthe target cell in response to the above-described start conditions (A1and A2).

In the second embodiment and the modifications thereof, the priority(cellReselectionPriority) may be considered as the priority assigned tothe frequency of the cell, or may be considered as the priority assignedto the cell. For example, the priority (cellReselectionPriority) may bethe priority of an inter-frequency, or may be the priority of aninter-RAT frequency cell.

Although not particularly mentioned in the twentieth through the twentysecond modification of the second embodiment, a method of reselectingthe target cell is not particularly restricted, and may be the methoddescribed in the embodiment or the other modifications.

Although not particularly mentioned in the second embodiment, a programmay be provided for causing a computer to execute each process performedby the UE 100 and the eNB 200. Furthermore, the program may be recordedon a computer-readable medium. If the computer-readable medium is used,it is possible to install the program in a computer. Here, thecomputer-readable medium recording therein the program may be anon-transitory recording medium. The non-transitory recording medium mayinclude, but not be limited to, for example, a recording medium such asa CD-ROM or a DVD-ROM.

Alternatively, a chip may be provided which includes: a memory in whicha program for performing each process performed by the UE 100 and theeNB 200 is stored; and a processor for executing the program stored inthe memory.

In the second embodiment, the LTE system is described as an example of amobile communication system. However, the embodiment is not limitedthereto. The mobile communication system may be a system other than theLTE system.

[Additional Statement 1]

(Instruction)

the new work item is approved on Multicarrier Load Distribution of UEsin LTE and the objective of this work item (WI) is to enhance the cellreselection mechanism in IDLE under up-to-date multicarrier operations.

The WI should first have a study phase to look at following.

-   -   Limitations of the current mechanisms and measurement quantities        for redistribution of UEs amongst multiple LTE carriers.

Based on the analysis of the study phase, the WI should providesolution(s) for following.

-   -   To redistribute RRC Idle UEs amongst LTE carriers that minimize        the need for load triggered HO or redirection of UE during        connected mode    -   Carriers with different cell load, bandwidth and capabilities        should be considered.    -   Both homogeneous and heterogeneous deployment scenarios should        be considered    -   New measurement quantities, e.g. SINR, for better estimation of        user throughput should be evaluated and introduced, if needed

In this additional statement, the potential issues in the cellreselection mechanism for multi-carrier load distribution are identifiedin support of the study phase.

(Deployment Scenarios)

FIGS. 14A to 14C are a diagram illustrating deployment scenarios. TheWID and the motivation document point out the examples of multi-carrierdeployment scenario, such as homogeneous coverage among multiplecarriers (inter-frequency HomoNet), heterogeneous coverage betweenmultiple carriers (inter-frequency HetNet), and the combination of thesetwo scenarios. Although a unified solution applicable to all threescenarios is desirable, scenario-specific solutions should also beconsidered if significant benefits can be realized. Therefore, anyproposed solution should be evaluated based on its applicability tospecific deployment scenario(s) and whether it causes degradation (e.g.,ping-ponging in reselection) in the non-intended deployment scenarios.

The solutions should be applicable to at least one deployment scenarioin inter-frequency HomoNet, inter-frequency HetNet or the combination ofthe two scenarios, and the solutions should not have negative impact onthe non-intended scenarios.

(Limitation in the Existing Cell Reselection Mechanism)

The cell (re)selection parameters are provided by SIBs or dedicatedsignalling (RRC Connection Reject or RRC Connection Release messages)and the cell reselection procedure is performed in the UE using theparameters. Focusing on normal cell reselection (i.e., without dedicatedparameters), SIB3 provides common parameters, while SIB4 providesintra-frequency specific parameters and SIB5 provides inter-frequencyspecific parameters. With these parameters, the UE performs frequencyprioritization according to the cell reselection priority, themeasurements of RSRP/RSRQ, the evaluations of the S-criterion, and theranking of cells with the R-criterion, and then it finds out the mostsuitable cell to reselect and camp on.

In the prioritization and measurement phase, the UE may not performintra-frequency measurement when the S-criterion of the serving cell isfulfilled. In addition, the UE may not perform inter-frequencymeasurement unless the reselection priority of the neighbour frequencyis higher than that of the serving frequency or the S-criterion of theserving cell is not fulfilled. It is obvious that the cell resectionprocedure is not triggered unless the measurement is performed, even ifmore suitable cell is actually available. It is one of limitations inthe current cell reselection procedure and causes lopsided UEdistribution in some cases, e.g., “lock-on” with idle mode mobilityillustrated in FIG. 15. FIG. 15 is a diagram illustrating “Lock-on”within higher priority frequency

In addition, there are exceptions in the prioritization phase. The UE isallowed to deviate from the normal frequency prioritization to selectionrule and select a different frequency that provides MBMS services ofinterest. Moreover, Rel-12 ProSe direct communication introduced asimilar rule that “If the UE capable of ProSe direct communication isconfigured to perform ProSe direct communication and can only performthe ProSe direct communication while camping on a frequency, the UE mayconsider that frequency to be the highest priority”. Therefore, it islikely that cells on frequencies offering MBMS and/or ProSe experiencecongestion more often than those that do not, if there are many UEsinterested in these types of services.

Note that it is assumed that the cell reselection for inter-RAT and CSGcell is out of the scope of this work item.

RAN2 should consider how load balance may be achieved between a lowpriority frequency and a high priority frequency with the understandingthat the UE is also allowed to prioritize a frequency based on itsinterest in MBMS and/or ProSe.

In the evaluation and ranking phase, both the S-criterion and theR-criterion use RSRP and RSRQ which are measured by the UE. However, itwas pointed out in that RSRQ is not a sufficient metric for determiningSINR since RSRQ has a narrow dynamic range and it becomes a non-linearfunction of SINR above 5 dB and is saturated around 10 dB. However, thethroughput is kept almost linear between 0 dB and 30 dB. The limitationwith RSRQ measurement is particularly troublesome for higher categoryUEs (e.g., smartphones) since this may degrade the achievable userthroughput. Therefore, in order to maximize achievable user throughputafter transitioning to RRC CONNECTED, at least the ranking process usingRSRQ is no longer adequate and a new measurement metric, with a betterestimate of SINR is desirable.

At least the ranking process for the higher category UEs should be basedon a more accurate measurement metric than RSRQ.

Unlike smartphones, the MTC devices with lower UE category don't needtoo much higher SINR, since the throughput ends up hitting the limitscapped by the Maximum number of DL-SCH transport block bits receivedwithin a TTI. Especially for such lower category UEs, it should be takeninto consideration to avoid unnecessary power consumption due to theincreased measurements using the new metric, as stated in the WID.

RAN2 should avoid additional power consumption for lower category UEs.

(Potential Issues and Challenges)

In this section, the potential issues, other than the limitationsidentified in the above section, are discussed.

(Potential Capacity (Static)/Cell Load (Dynamic) Awareness Distribution)

The potential capacities of frequencies/cells are different since cellshave independent configuration of parameters such as bandwidth, CPlength, almost blank subframes (ABS), MBSFN subframes, ProSe resourcepools, number of antennas and/or cell sizes depending on transmissionpower of eNB and operating frequency. These can be categorized intofrequency-domain, time-domain and space-domain (illustrated in FIGS. 16Ato 16C). FIGS. 16A to 16C are a diagram illustrating Potential capacityand (semi-) static configurations. The potential capacity of a cell maybe determined as a function of the above parameters, and since theseconfigurations are typically (semi-) static and are already provided inSIBs, they may be acquired by IDLE UEs through UE implementation orassistance from their serving cells. Since a cell's capacity is directlyrelated to the potential for user throughput UEs should take such cellcapacity into account as part of cell reselection.

The cell reselection procedure should take into account theconfigurations of neighbour cells.

As intended in, more dynamic information such as current cell loads willfacilitate more suitable cell reselection; however, the direct broadcastof load information is not acceptable by some operators based onprevious discussions e.g., Rel-12 WLAN Interworking. As in the case forWLAN Interworking, the cell loads are implicitly provided through theconfigurations of RAN assistance parameters, i.e., RSRP/RSRQ thresholds.Since a cell's load is an integral part of idle mode UE distribution, itshould be considered whether implicit or direct configuration of acell's load should also be considered for optimizing cell reselection

RAN2 should discuss whether it can be acceptable to provide dynamicinformation such as current cell load to IDLE UEs, directly orimplicitly.

(Clustered UE Distribution)

In general, it is well-known that the density of users is not uniform inan area and the user tends to cluster within specific spots, e.g., in abuilding, on a main street, in a stadium and so on. Therefore, thedistribution of UE clusters among multiple carriers within an eNBcoverage area may be one of the keys to successful load distribution.

With connected mode UEs, load balancing may be resolved using existingmechanisms such as redirections or handovers. On the other hand, idlemode UEs basically applies the common configuration provided by SIBs.Assuming the clustered UEs experience similar radio conditions, i.e.,RSRP and RSRQ, the current cell reselection mechanism cannot split thecluster, i.e., these UEs may reselect based on the sameS-criterion/R-criterion. So, even if cell reselection parameters areadjusted by the eNB, it simply results in “mass reselection” and loadbalancing among multiple carriers cannot be achieved (as illustrated inFIGS. 17A and 17B). FIGS. 17A and 17B are a diagram illustrating themass reselection of the clustered UE. To avoid the mass reselection andachieve better load balancing, it should be possible for a group of UEswithin the cluster to select cell(s) that differ from cell(s) selectedby other group of UEs within the cluster. It should be furtherconsidered whether it would be beneficial for the cells to be reselectedusing some kind of UE-by-UE randomization scheme. Such a randomizationscheme could be based on an acceptable measurement range e.g., CellReselection Priority and/or R-criterion, where the measurement range isunder control of the eNB. Therefore, RAN2 should consider how to achieveload balance of the clustered UEs.

RAN2 should consider how to achieve load balancing of the clustered UEsamong multiple carriers.

[Additional Statement 2]

(Introduction)

In the present additional statement, the two mechanisms are identifiedfor the multi-carrier load distribution, i.e., the continuousdistribution and the one-shot re-distribution, and the solutions withper-cell parameter and/or reselection probabilities are considered.

(Pre-Conditions and Post-Conditions)

Before considering the solutions, it's worth revisiting why the idle UEsmay be distributed non-uniformly among multiple carriers today. FIG. 18illustrates three possible pre-conditions, i.e., (a) to (c), and apost-condition, i.e., (d), assuming four frequency layers for thepurpose of coverage and capacity. F1 is the lowest frequency (e.g., 800MHz, thus it has larger coverage) and F4 is the highest frequency (e.g.,3.5 GHz, thus it has smaller coverage). It is noted in FIG. 18 that thecondition (b) has reverse order compared to the other conditions. In thecondition (a) and (b) each frequency layer has differentcellReselectionPriority, while the condition (c) intends equal priorityamong all frequencies.

The condition (a) may be considered as a typical priority configuration,whereby the small cell layers on higher frequencies are prioritized forreselection to achieve capacity enhancements. The UEs are camped on F4or F2 as long as the S-criteria of the cell is fulfilled, since thecurrent specification forces the UE to perform inter-frequencymeasurements and cell reselection to a cell configured with higherpriority frequency than the frequency of the serving cell, regardless ofSqual/Srxlev of the serving cell.

The condition (b) may be considered for the coverage-prioritizedconfiguration e.g., in Dual Connectivity capable network. This resultsin a worse condition than condition (a), since all UEs are camped on F1.

It should be noted that the cellReselectionPriority may cause loadimbalance of idle UEs beyond just two frequency layers, i.e., imbalancebetween F1 and F2, or between F3 and F4 in FIG. 18. Although the currentmechanism allows configuring different ThreshX, HighQ/ThreshX, HighP foreach frequency priority, it cannot solve the distribution issue since achange of these parameters cause mass reselection and agreement“Solution should be able to move fraction of the UEs from one cell toanother cell” above cannot be met. Even if the cell specific priority isprovided in Rel-13, the prioritized cell may experience higher load dueto the mass reselection (also known as “surge of UEs” or “closelylocated UEs problem”) assuming the cells offers the same capacity (e.g.,the same bandwidth), although the solution may work well in a specificscenario, e.g., reselection to small cells on co-channel HetNet layer.

The cell reselection prioritization itself may cause load imbalanceamong multiple frequencies, regardless of whether it's on a frequencylayer or a cell layer.

Condition (c) configures all frequency layers with the same priority,i.e., equal priority inter-frequency case. With equal priorityinter-frequency, the cell ranking is performed by R-criteria and the UEshould select a cell with the best RSRP, taking into account of theQoffset to compensate for different pathloss due to differentfrequencies. As in the case for conditions (a) and (b), the currentmechanism also cannot meet the agreement that Solution should be able tomove fraction of the UEs from one cell to another cell. However, thecurrent cell ranking mechanism is useful in solving the issue “Maximizeuser throughput and network capacity (in terms of system throughput,connection establishment, RA, (inter-frequency) mobility relatedsignalling) for UEs in CONNECTED”, since the UE can reselect the cellthat offers the best radio condition, although it only relies on RSRP.

The equal frequency priority with cell ranking has a potential tomaximize user throughput and network capacity.

The condition (d) is an example of a desirable post-condition, whereinthe UEs are camped on each frequency/cell uniformly and all cells arenot heavy-loaded. The post-condition in (d) is consistent with theagreement above that a Solution should be able to move fraction of theUEs from one cell to another cell which may be using e.g. per-cellparameter and/or reselection probabilities.

The most significant benefit of the enhanced continuous distributionmechanism such as CSP is to allow the cells to remain lightly-loaded,i.e., the network's UE distribution may be maintained and used toprevent the pre-conditions (a)-(c) in FIG. 18 from occurring. On theother hand, one of the main drawbacks with the enhanced continuousdistribution is the impact to the power consumption of IDLE UEs eventhough the network does not suffer from any over-load conditions, assuggested. If further solutions such as the cell-specific priority withprobability and/or with random threshold offset are introduced, the UEpower consumption may be substantially increased. Additionally,operators may face challenges to modify their existing networkreselection mechanism even if the operator does not suffer from thenecessity redistribute idle UEs.

The enhanced continuous distribution mechanism is expected to have moreimpacts on the UE power consumption and the configurations of theexisting network deployments.

If an enhanced continuous distribution is introduced to fulfil theagreement that 4) It should be possible to control the load distributionamong individual cells rather than only on a carrier level (for examplethe scenario that the macro cell in a co-channel Het-Net deploymentand/or certain small cells on another carrier may be overloaded), itshould not require drastic changes to network deployment, but simply aimto provide more flexibilities based on the existing mechanism.

Proposal 1: The existing reselection mechanism or the small enhancementmay work well, other than some heavily loaded network deploymentscenarios.

Even if the enhanced continuous distribution could statisticallymaintain the network under light load, the traffic demands, which iscorrelated with the density of IDLE UEs in an area, could varysignificantly e.g., at sports events, during commuting hours, within ashopping mall in weekends or under emergency conditions. In such aheavy-loaded condition, there should be a means for the network toreturn to a more balanced loaded condition as soon as possible, by meansof a one-shot re-distribution mechanism. Once the one-shotre-distribution mechanism allows the network to return to a normal loadcondition, the enhanced continuous distribution may be activated againto maintain the network under the balanced load condition.

The one-shot re-distribution mechanism may be useful under heavilyloaded network due to sudden surge in traffic demand.

Since the one-shot re-distribution mechanisms is mainly used for casesof heavily loaded network due to sudden surge in traffic demand, it maybe applied independently of the existing mechanisms and thus it will notconflict with the current network strategies. Furthermore, the impactson UE power consumption would be reduced since the power-consumed eventis performed only once. The network may initiate the one-shotre-distribution when it experiences higher load in multiplecells/frequencies, i.e., the preconditions (a)-(c) in FIG. 18. Inaddition, the word “re-distribute” within agreement 1) above isconsistent with the need for a one-shot re-distribution mechanism, sincethe continuous distribution or enhanced continuous distribution shouldnot be depicted by as a means to “re-distribute” the UEs.

Proposal 2: RAN2 should consider the one-shot re-distribution mechanismas an independent mechanism from the continuous distribution or enhancedcontinuous distribution mechanism.

(Enhanced Continuous Distribution Mechanism)

For the extreme case when the network is lightly loaded and there isonly one UE camped on the macro cell, there is no reason for the UE tostay on the macro cell layer when a small cell fulfils a threshold forthe UE. In this case, either the prioritization mechanisms, or theexisting frequency priority or the additional cell-specific priority(CSP), still works well. With the CSP which was already a baseline, thenetwork is offered more flexibility in its configuration with minimumimpacts on the existing rules for the priority handling. This is similarapproach with the agreements for IncMon to Extend the number of cellreselection priorities to reduce number of reselections between equalpriority carriers, from the configuration flexibility point of view.Therefore, the enhanced continuous distribution based on the existingcell reselection mechanism should only have an additional means toconfigure priorities on a cell basis with CSP, although it should benoted that it potentially has a drawback which may lead to loadimbalance when the number of UEs increases. In this case anothersolution may be needed, i.e., the one-shot re-distribution mechanism.

Proposal 3: RAN2 should conclude for lightly/medium loaded network thatno additional mechanism is necessary in the existing cell reselectionmechanism, except for optional provisioning of the cell-specificpriority and the already agreed extended reselection priorities.

(Benefit of One-Shot Re-Distribution Mechanism)

Trigger for Re-Distribution

The trigger for re-distribution, i.e., when the UE initiates/continuesthe (special) cell reselection procedure, may be based on one of thefollowing options.

Option 1: When the network broadcasts parameters for there-distribution;

Option 1-a: When the network provides the list of cell-specificpriorities;

It's assumed as the trigger for the cell-specific priority withrandomization as well as CSP itself. The UE should continue the cellreselection procedure, including inter-frequency measurements, as longas the cell-specific priority provided is higher than the priority ofserving cell, as it is today.

Option 1-b: When the network provides the list of cell-specificprobabilities;

This option assumes the trigger for reselection is based on thecell-specific priority with probability (CSPP). The UE should generate arandom value and perform the cell reselection procedure when the randomvalue is over the cell-specific priority provided.

Option 1-c: When the network provides the maximum value of reselections;

This option assumes the counter-based scheme discussed. The UE shouldcount the number of cell reselection performed and ignore thehigher-prioritized cell when the count is over the maximum valuebroadcasted by the serving cell.

Option 1-d: When the network updates the parameters;

It's assumed the trigger is implied by the updated parameters in SIB orthe paging. The UE should apply the updated parameters and perform thecell reselection procedure.

Option 2: When the network provides an explicit request;

This option assumes a pure trigger for re-distribution. The UE appliesthe (special) parameters and perform the cell reselection procedure onlyonce upon reception of the request. The request may require 1-bitsignalling in either a SIB or in the paging message as an application.The UE may only monitor the request since the parameter for the(special) cell reselection can be broadcasted in advance.

Option 3: When the serving cell configures dedicated parameters over RRCConnection Release;

It is for the specific case the UE transitions from RRC Connected toIdle and the RRC Connection Release may have the additionalcell-specific offsets, the extra grouping, and/or the existing dedicatedpriority.

Options 1-x are based on typical mechanisms for reselection control, butthese have some drawbacks. With Options 1-x, the UE is forced to applyadditional continuous inter-frequency measurements during the parametersare provided, e.g., if the cell prioritized by Option 1-a is not on thefrequencies prioritized by the existing cellReselectionPriority. Itresults in additional UE power consumption and it should be avoided.Also, it is not crystal clear at which points in time the UE shouldre-calculate the probability.

Additionally, Options 1-x may require the UE to monitor continuously tocheck whether the parameters are updated, or whether the network decidedto initiate a (special) cell reselection procedure. Option 2 is a simplesolution for the one-shot re-distribution mechanism, although it doesrequire 1-bit signalling to inform UE of the activation of thisprocedure. Option 3 may also be an nice way if the solution only relieson the dedicated signalling, but it cannot fulfil the requirement toMaximize user throughput and network capacity (in terms of systemthroughput, connection establishment, RA, (inter-frequency) mobilityrelated signalling) for UEs in CONNECTED. Therefore, Option 2 is thepreferred solution for the trigger for re-distribution.

Proposal 4: The one-shot re-distribution should be triggered by anexplicit request indication broadcasted by the serving cell.

Proposal 5: If the Proposal 4 is agreeable, RAN2 should discuss whetherthe indication is provided in SIB or the paging.

[Additional Statement 3]

(Introduction)

The present additional statement considers the solutions with per-cellparameter and/or reselection probabilities for the one-shotre-distribution mechanism.

(One-Shot Re-Distribution Mechanism)

The one-shot re-distribution obviously needs a trigger for theinitiation of special cell reselection procedure. The followingsolutions for cell reselection by fraction of UEs are considered afterthe triggering.

(Cell Reselection by Fraction of UEs)

It is one of big challenges in this WI that Solution should be able tomove fraction of the UEs from one cell to another cell. The possiblesolutions were suggested as follows.

Assuming the cell-specific priority list is provisioned in advance andthe priority has to be higher than that of the serving cell.

Option 1-a: Cell-Specific Priority with Random Threshold Offset [4]

If it could be applicable to the one-shot re-distribution, when atrigger happens the UE generates the random value for a cell of higherpriority frequency, and the UE reselects the cell if Squal>Thresh′X,HighQ or Srxlev>Thresh′X, HighP is fulfilled, wherein Thresh′X, HighQ orThresh′X, HighP is applied with the randomized offset (i.e., Thresh′X,HighQ=ThreshX, HighQ+offset x/cell*random).

One drawback with this approach is that the Thresh′X may not ensure gooduser throughput with wider/negative range of the randomized offset andthat only UEs in cell-edge can move to the other cell with narrowerrange of the randomized offset.

Option 1-b: Cell-Specific Priority with Probability (CSPP)

If it could be applicable to the one-shot re-distribution, when atrigger happens the UE generates the random value for the cellprioritized and reselect it if the random value exceeds the probabilityprovided.

One drawback may be that the cell reselected can only ensure S-criteria,not for ThreshX nor R-criteria. It may result in the degradation of userthroughput when the UE transitions to RRC Connected.

Option 1-c: Counter-Based Scheme with Number of Cell Reselection

If it could be applicable to the one-shot re-distribution, whereby theUE keeps track of the number of reselections and when a trigger happens,the UE decides whether the cell reselection priority should be appliedor ignored based on the counter value.

Independent of the priority handling, i.e., not only for higher prioritycells.

Option 2-a: Ranking Randomization within a Specified Range

This option is a kind of extension on Option 1-b (from the randomizationpoint of view) and essentially similar approach with Option 1-a (fromthe range concept perspective), but Option 2-a focuses on theenhancements in the ranking process. With the specified range, thedegradation of user throughput may be limited and is under networkcontrol. The details of range are FFS, e.g., the unit may be [dB].

Option 2-b: Ranking-Specific Probability

This option is basically similar approach with Option 2-a, but Option2-b focuses on the enhancements in the ranking process withprobabilities similar to Option 1-b. Since the probability iscorresponding to each rank, the UE is allowed to reselect the cell withbetter rank order and probabilities.

All options could move fraction of the UEs from one cell to anothercell. The most significant difference between Options 1-x and Options2-x is whether a cell specific priority list (CSP) is needed or not.Since Options 1-x would only take the priority handling in thereselection consideration, the reselected cell may not be (sub-)optimalfor each UE from the radio quality point of view, i.e., the new cell mayonly fulfil the S-criterion even if some cells offering better radioquality are available around the UE. In addition, Options 1-x cannotconsider equal priority frequencies/cells since these options rely onthe configuration with higher priority frequencies/cells.

Options 2-x may be based on the existing ranking process which ensured acell reselection to a better cell as the result of comparison to cellsthe UE detects. So, Options 2-x can potentially Maximize user throughputand network capacity (in terms of system throughput, connectionestablishment, RA, (inter-frequency) mobility related signalling) forUEs in CONNECTED. Therefore, the one-shot re-distribution should adaptthe ranking process.

Proposal 1: The ranking process should be enhanced for the one-shotre-distribution.

(Target Cells/Frequencies for Enhanced Ranking Process)

If the Proposal 1 is agreeable, the issue is which cell/frequency shouldbe considered in the enhanced ranking process. In the currentspecification, the evaluation with the R-criteria applies forintra-frequency and equal priority inter-frequency cells. So, tominimize changes in the specification, the target cells/frequencies forthe enhanced ranking process should also be considered under equalpriority cells/frequencies.

Proposal 2: The target cells/frequencies in the one-shot re-distributionshould only be applicable under equal priority.

If the Proposal 2 is acceptable, it is necessary to determine how theequal priority target cells/frequencies are provided to the UE for theone-shot distribution. The following alternatives could be considered.

Alt.1: The serving cell provides the cell/frequency list explicitly.

The UE simply consider the cells/frequencies provided in the list in theenhanced ranking process.

Alt.2: The serving cell provides the cell/frequency implicitly.

It could use the extended cell reselection priority. For eachcell/frequency configured with the extended cell reselection priority,the UE determines these cells/frequencies for enhanced ranking process.It is FFS whether the UE should also take the legacy cell reselectionpriority into account or not, while some examples are suggested on Table1.

From the perspective of signalling overhead, Alt.2 is better than Alt.l.However, Alt.2 will need at least one more rule to determine the targetcells/frequencies, e.g., whether the prioritized cells/frequencies inthe legacy cell reselection priority should still be prioritized.Although pros and cons can be seen in both alternatives, Alt.2 isslightly preferred solution.

Proposal 3: RAN2 should decide if the cells/frequencies configured withthe extended cell reselection priority should be considered as equalpriority in the one-shot re-distribution.

Table 1 shows examples of implicit target cells/frequencies provisioningwith Alt.2

TABLE 1 Legacy priority Extended priority i.e., e.g.,cellReselectionPriority CellReselSubPriority-r12 [10] [11] Alt.2-1Alt.2-2 Alt.2-3 Alt.2-4 F1 7 3 Target Target Target Target F2 7 3 TargetTarget Target Target F3 7 1 Target Target F4 7 (not provided) F5 3 3Target Target F6 3 1 Target F7 3 (not provided)

Note: The serving cell may be legacy priority 5, and the targetcells/frequencies are configured.

Alt.2-1: a (e.g., higher) legacy priority and any extended priority areused.

Alt.2-2: a (e.g., higher) legacy priority and a (e.g., higher) extendedpriority are used.

Alt.2-3: any extended priority, regardless of the legacy priority isused.

Alt.2-4: a (e.g., higher) extended priority, regardless of the legacypriority is used.

(Additional Considerations)

Further Considerations for Randomization

It was pointed out that the probability-based scheme has some concern onthe testability and/or controllability of the reselection by thenetwork. Thus, the counter-based scheme was suggested as an alternativeto the probabilistic-based scheme. However, if the probability-basedscheme is really problematic, it could also be considered whether therandomization may be based on the UE_ID (or the IMSI) similar to theexisting paging frame/occasion determination. For example of IMSI, theUE could perform the (special) cell reselection procedure if (IMSI modNp)=Nr is fulfilled, wherein Np and Nr configured based on the expectedprobability and fairness among UEs. If Np and Nr are configured with 10and 0 respectively, the probability fulfilling the formula is 10%. Ifthe IMSIs are assumed adequately randomized among UEs within a cell, thenetwork may control the probability, while the UE testability would bemuch simpler.

Proposal 4: If the probability-based scheme is not preferred, theIMSI-based randomization should be one of potential methods instead.

Possibility of RSRQ/SINR-Based Ranking Process

The current R-criteria only consider RSRP, thus any radio quality arenot taken into account. If only received power is evaluated, it couldensure the optimal throughput in homogeneous network with macro cells.However, today it cannot be true with HetNet deployments, e.g., thesmall cell located near a macro cell may offer better RSRP but worseRSRQ than a cell on the frequency layer with small cell only. Tomaximize the user throughput and network capacity when the UEtransitions to Connected, it's worth revisiting the R-criteria with RSRQor SINR. If IDLE UEs uses a more accurate indication of signal qualitythan what is used today, it would potentially meet the requirement toMaximize user throughput and network capacity (in terms of systemthroughput, connection establishment, RA, (inter-frequency) mobilityrelated signalling) for UEs in CONNECTED.

Proposal 5: RAN2 should consider whether the R-criteria should berevisited using a different signal quality criteria considering HetNetdeployments.

[Additional Statement 4]

(Introduction)

The merged solution for Continuous Re-distribution Scheme (CRS) and OneShot Scheme (OSS) was proposed and technically endorsed for idle modedistribution. The merged solution reuses overall re-distributionmechanism of the harmonized solution for CRS with IMSI-basedre-distribution.

In the present additional statement, the merged solution is verifiedfrom the OSS's perspective along with one possible optimization toreduce NW/UE complexity.

(Merged Solution with the OSS Option)

Two scenarios were discussed, i.e., long term unbalanced distribution(Scenario 1) and short term unbalanced distribution (Scenario 2). It wasidentified that OSS is useful especially for Scenario 2. The followingagreements were reached.

Agreements

1. Use followings as the baseline of OSS:

Paging may contain a simple reprioritization request to instruct theUE/UEs currently used/prioritized carrier should be temporarily assigned(with a timer) the lowest priority.

2. The solution in R2-154729 as the baseline of CRS. But how to selectUEs for the portion of moving to another layer is still FFS.

3. Both CRS and OSS solution should be introduced for differentscenarios.

4. Both details of the two solutions should be further identified.

Upon further offline discussions, it was agreed that agreements 3 and 4,can be combined into a merged solution incorporating both therequirements for the OSS solution (agreement 1) and the CRS solution(agreement 2). The merged solution works very well especially underScenario 1 or if both scenarios, i.e., Scenario 1 and Scenario 2 areneeded in succession.

Observation 1: The merged CRS and OSS solution is optimized for thenecessity to support Scenario 1 or if both Scenario 1 and Scenario 2 areneeded in succession.

On the other hand, if the operator only needs to support Scenario 2 at agiven time, it may be assumed that only OSS is needed. According to theprocedure of the merged solution, the OSS operation is achieved with thefollowing five steps.

(1) SI update to provide the redistribution parameters with the optionto trigger OSS applicable.

(2) UE acquires the redistribution parameters in SIB3 and SIB5.

(3) Selective UEs that receive the OSS paging message trigger/apply theredistribution parameters,

(4) UE performs the redistribution evaluation/reselection. And,

(5) SI update at the next modification boundary to remove theredistribution parameters and apply the legacy parameters.

It above procedure does not capture the intention of agreement 1 basedon the proposed OSS solutions, which requires only two simple steps asfollows.

a) OSS is triggered for the UE upon receiving the OSS paging message.

b) UE performs the redistribution evaluation/reselection based ondeprioritization of the serving cell/frequency.

Considering the load redistribution is often needed under the congestedcase, it's would be a disadvantage to tolerate the additional signallingoverhead required of the merged solution.

Observation 2: The merged CRS and OSS solution requires excessive stepsand signalling overhead if only the OSS operation is needed.

In addition, the SI update requires that all UEs re-acquire the updatedsystem information, regardless of whether the UEs are later triggered bythe OSS paging message i.e., the updated system information isusefulness for those UEs which are not targeted for the OSS. It willresult in extra UE power consumption which could be avoided if agreement1 were realized through the two steps (e.g., steps a and b above) sinceonly a fraction of UEs are triggered for OSS. So, the merged solutionshould also be optimized for the case when only Scenario 2 is needed.

Observation 3: If only OSS operation is needed, the merged solutionunnecessarily forces all the UEs to update SI resulting with increasedUE power consumptions.

Proposal 1: The merged CRS and OSS solution should be optimized also forScenario 2, i.e., OSS-only operation should be possible with two stepsand without SI update.

(Possible Optimization for OSS-Only Operation)

If Proposal 1 is acceptable, it could be straight forward to adopt theagreement 1 that “Paging may contain a simple reprioritization requestto instruct the UE/UEs currently used/prioritized carrier should betemporarily assigned (with a timer) the lowest priority”, into themerged solution. Since the merged solution already has the OSS pagingmessage and the timer, the issue is how “the UE/UEs currentlyused/prioritized carrier should be temporarily assigned [ . . . ] thelowest priority”.

According to the merged solution, the redistribution parameters areoptional. It is assumed that when the parameters are not provided inSIB3/5, the UE performs the legacy cell reselection procedure(regardless of the setting of OSS applicable), wherein the mergedsolution assumes the SI does not include the optional redistributionparameters as shown in Table 2.

TABLE 2 OSS applicable [1] 0 (OFF) 1 (ON) Redistribution Not (Legacycell (Legacy cell parameters available reselection) reselection)(optional) Possible target Possible target i.e., for OSS for OSSredistributionFactor/ optimization optimization redistributionTimerAvailable CRS OSS with paging and trigger interFreqNeighCell List2 [1]

Observation 4: The UE performs the legacy cell reselection procedurewhen the redistribution parameters are not provided in SIB3/5.

During the legacy cell reselection procedure, i.e., when the optionalredistribution parameters are not provided (as highlighted in yellow inTable 1), the UE is still able to receive the paging message, althoughthe UE is not expected to perform OSS without the SIB updates in themerged solution. If this kind of OSS paging message is allowed, thesimplest optimization should be that the UE just considers the currentcell/frequency as the lowest priority and performs cell reselectionaccording to the legacy procedure. To align with the redistributionevaluation in the merged solution, it can be rephrased that the UEconsiders all neighbour cells/frequencies as having higher priority thanthe serving cell/frequency.

This optimization will minimize the signalling overhead compared to thecurrent OSS operation with the merged solution, i.e., no SI update andno broadcast of the redistribution parameters. Regarding the additionalpower consumption, it affects only on a fraction of UEs which are paged,i.e., the rest of UEs do not suffer from any power consumption for theOSS operation. Also, the impacts on the merged solution and/or thelegacy specification are quite small. Therefore, RAN2 should enhance themerged solution with this “add-on” optimization.

Note that if the optional redistribution parameters are provided and OSSapplicable is set to “1”, the UE should continue to follow the procedurein the merged solution without the optimization.

Proposal 2: If the OSS paging message is received and the redistributionparameters are not provided in SIB (regardless of OSS applicable bit),the UE should perform the legacy cell reselection procedure with theexception that prioritization of all neighbour cells/frequencies will behigher than that of the serving cell/frequency.

(Annex: Potential Change for the OSS Optimization) The potential changein the specification for the OSS optimization is as follows.

Start of Change

5.2.4: Cell Reselection evaluation process

5.2.4.1: Reselection priorities handling

Absolute priorities of different E-UTRAN frequencies or inter-RATfrequencies may be provided to the UE in the system information, in theRRCConnectionRelease message, or by inheriting from another RAT atinter-RAT cell (re)selection. In the case of system information, anE-UTRAN frequency or inter-RAT frequency may be listed without providinga priority (i.e. the field cellReselectionPriority is absent for thatfrequency). If priorities are provided in dedicated signalling, the UEshall ignore all the priorities provided in system information. If UE isin camped on any cell state, UE shall only apply the priorities providedby system information from current cell, and the UE preserves prioritiesprovided by dedicated signalling and deprioritisationReq received inRRCConnectionReject unless specified otherwise. When the UE in campednormally state, has only dedicated priorities other than for the currentfrequency, the UE shall consider the current frequency to be the lowestpriority frequency (i.e. lower than the eight network configuredvalues). Also if the UE receives an OSS paging message, and if theSystem Information does not contain any redistribution parameters (i.e.the field redistributionFactor is absent for all frequencies/cells) orthe UE only has dedicated priorities other than for the currentfrequency, the UE shall consider the current frequency/cell to be thelowest priority frequency/cell. While the UE is camped on a suitable CSGcell, the UE shall always consider the current frequency to be thehighest priority frequency (i.e. higher than the eight networkconfigured values), irrespective of any other priority value allocatedto this frequency. If the UE capable of sidelink communication isconfigured to perform sidelink communication and can only perform thesidelink communication while camping on a frequency, the UE may considerthat frequency to be the highest priority.

NOTE: The prioritization among the frequencies which UE considers to bethe highest priority frequency is left to UE implementation.

[Additional Statement 5]

(Introduction)

RAN2 agreed the direction of solution for the multi-carrier loadre-distribution.

Agreements

1. Following Requirements can't be met by existing cell reselectionscheme.

1) It should be possible under network control to re-distribute amongthe different carriers a fraction of users currently camped on thesecarriers.

4) It should be possible to control the load distribution amongindividual cells rather than only on a carrier level (for example thescenario that the macro cell in a co-channel Het-Net deployment and/orcertain small cells on another carrier may be overloaded)

2. Solution should be able to move fraction of the UEs from one cell toanother cell.

3. To focus on solutions using e.g. per-cell parameter and/orreselection probabilities from RAN2#91 meeting.

RAN2 further agreed to “Adopt cell specific priorities as baseline”.This could fulfil the requirement 4) above. However, the solutions tore-distribute a fraction of users, i.e., the requirement 1), could notbe converged within the last meeting.

In the present additional statement, a possible compromise solution forthe requirement 1) is discussed.

(Randomization Mechanisms)

Current Solutions

To fulfil the requirement 4), especially to re-distribute a fraction ofusers, it's considered that some sort of randomization mechanism wouldbe useful since the existing priority itself cannot avoid themass-reselection. With regards to the randomization mechanisms, possiblesolutions have been already proposed, such as the randomized thresholdoffset, CSPP, CSPP with best-cell and the ranking randomization. Theyall share the commonality that the UE generates the random value todetermine which UE should perform the enhanced cell reselection, thusall these mechanisms may be considered as UE-based mechanisms. Theadvantage of UE-based mechanisms would allow the Idle UEs to move to adifferent frequency according to a probability, all the time. Onedrawback may be that it's unclear when the UE should stop the loadre-distribution process, which may result in unstable probability due tomultiple triggering of the reselection procedure.

That is, UE-based mechanism with random value, can re-distribute IdleUEs, while it may result in unstable probability due to multipletriggering of the reselection procedure.

On the other hand, the solution using dedicated signalling is alsoproposed, which offers tighter network control, thus it could beconsidered as NW-based mechanism, although it may “be insufficient toaddress all Idle mode Load Balancing needs”.

NW-based mechanism, i.e., with dedicated signalling, ensures tighternetwork control, while it's insufficient to re-distribute all Idle UEs.

RAN2 has four alternatives to move forward as follows;

Alt.1: Adopt only the UE-based solution.

Alt.2: Adopt only the NW-based solution.

Alt.3: Adopt both NW-based and UE-based solutions in Rel-13.

Alt.4: Consider an alternative solution consisting of benefits from bothUE-based and NW-based solutions.

With regards to Alt.1 and Alt.2, although both UE-based and NW-basedmechanisms can potentially resolve the issues in requirement 4), each ofthe solutions have their benefits and drawbacks, as mentioned inObservation land Observation 2 respectively. Regarding Alt.3, thedifferent suboptimal solutions will be specified to resolve the oneissue. It causes more standardization efforts and also brings morecomplexity from the UE and NW implementation point of view, which shouldbe avoided as much as possible. With Alt.4, RAN2 is potentially able toconverge to a single solution based on the ideas, issues and preferencesdiscussed in prior meetings, with minimal specification impact.

RAN2 should consider a compromise solution to re-distribute a fractionof UEs among different carrier under network control, to take advantageof the benefits of current solutions.

(Compromise Solution: NW-Based Randomization of Idle UEs)

Randomization Scheme

The randomization scheme should fulfil the requirement “to re-distribute[ . . . ] a fraction of users currently camped on these carriers”.UE-based randomization relies on random value generated by the UE, whichallows a fraction of Idle UEs to move to another frequency with aprobability at any time. So, it's worth considering whether NW-basedapproach can achieve similar effect. Considering many UEs are located inan area, it could be assumed that the IMSIs of UEs are also randomlydistributed as illustrated in FIG. 19. It would be already seen as arandom value.

Assuming IMSIs are sufficiently randomized in any given geographicalarea, NW-based approach could perform randomization by means of RRCsignalling. The two possibilities could be considered as follows.

Option 1: NW-based randomization with paging occasions

-   -   It was suggested to use the paging message as a trigger of load        re-distribution. The paging message could be essentially the        signalling towards specific/deterministic UEs, which are equal        to “a fraction of users”. Since the paging frame/occasion of        each UE is determined by IMSI, the eNB can control the        probability that UEs move to another frequency, e.g., if the eNB        pages at 1/10 occasions of all paging occasions in the default        DRX cycle then 10% of UEs are triggered for load        re-distribution.    -   It is necessary to add an indication into the paging message to        notify the UEs of the load re-distribution. But it would be        1-bit.

Option 2: NW-based randomization with modulo calculation

-   -   It could be also possible to determine specific/deterministic        UEs with modulo calculation of IMSI. For example, the UE could        perform the load re-distribution if “(IMSI mod Np)=Nr” is        fulfilled, wherein Np and Nr configured based on the expected        probability and fairness among UEs. If Np and Nr are configured        with 10 and 0 respectively, the probability of fulfilling the        equation is 10%.    -   The parameters, i.e., Np and Nr in the example, may be provided        in the broadcast signalling.

In comparison with the UE-based mechanisms, these options have the sameadvantage in terms of controlling Idle UEs and use of random valueand/or probability. The most significant benefit with the options isthat it is no longer necessary consider when the UE should stop the loadre-distribution process since the NW selects specific/deterministic setof UEs as a fraction of all UEs, whereby once the re-distribution isperformed the UEs are no longer on the serving frequency. Although theseNW-based options can be readily seen as applicable to a one-shotmechanism, they may also be extended to a continuous distributionmechanism.

Note that these options can work even if all UEs (all IMSIs) are not ona frequency/cell as normally assumed.

Proposal 2: It is beneficial for the network to select a fraction ofusers based on UE's IMSI for load re-distribution to achieverandomization for targeted UEs.

Comparing the options, the accuracy of control is similar but thesignalling overhead and impacts on the existing specification wouldincrease with Option 2. In addition, Option 2 may need SI Update totrigger the load re-distribution. So, Option 1 with the paging messageis preferable.

Proposal 3: When the UE receives an indication in the paging message,the UE should perform the load re-distribution process.

(Load Re-Distribution Process)

The load re-distribution process should fulfil the requirement “tore-distribute among the different carriers”. The simplest way could bethat the UE considers the current frequency as the lowest priority onlywhen the load re-distribution is triggered. Since the lowest priorityfrequency is specified as “lower than the eight network configuredvalues”, the UE shall move to the other frequency as long as there is acell fulfilling the criteria, e.g., Squal>ThreshX, HighQ. It makes sensebecause it could be assumed the serving cell currently suffers fromheavy load.

Proposal 4: The UE should consider the current frequency to be thelowest priority frequency during the load re-distribution process afterthe trigger.

It was also suggested that as a requirement, it's also important tomaximize user throughput when the UE transitions to Connected. But itcould be solved by the continuous distribution, i.e., the existing cellreselection, without any enhancements in this WI. For example, after theUE moves to the second frequency due to the load re-distribution, ifthere is the same priority frequency on the third frequency layer withthe second frequency currently the UE camped on then the UE shouldperform the equal priority inter-frequency reselection and select thebest cell according to the R-criteria as it is today. So, it could beconsidered that the requirement to maximize user throughput is solved bythe network deployment with the existing equal priority reselection.

FIG. 20 is a diagram illustrating simple example with Option 2 andcombination approach with the existing mechanism.

Proposal 5: It should be assumed that the network deployment couldensure better user throughput, after the load re-distribution.

(One-Shot Mechanism and Continuous Distribution)

It was discussed whether the one-shot mechanism is beneficial, althoughit had been considered the enhancements in this WI was applicablecontinuously. The one-shot mechanism works well to normalize the networkload which is suddenly increased by e.g., the start of an eMBMS sessionor the some special hours such as a sports event. However, it may be notsufficient to have only the one-shot mechanism. Considering continuousmobility of idle UEs, it would be true that the load imbalance cannotavoid only with the existing cell reselection mechanism in somedeployment scenarios, e.g., whereby the UEs moving into the area alwaysreselects a specific layer depending on RSRP, i.e., pathloss differencebetween two macro cell layers, even if the network offers the macro celllayers with equal priority. So, it's worth considering whether thesolution can be applicable to both the one-shot mechanism and thecontinuous distribution.

The continuous distribution may be still necessary in some deploymentscenarios.

If the proposals in this additional statement are agreeable, thecompromise solution can be applied not only for one-shot re-distributionbut also for continuous distribution. If the eNB continuously triggersthe load re-distribution in a paging occasion, it could be actually thecontinuous distribution, i.e., a probability of UEs in an area (amongall frequencies) are moved from the cell triggering the re-distribution.The compromise solution can ensure stable probability even if the loadre-distribution is triggered twice or more. So, if RAN2 aims a singlesolution for different scenarios, it should focus on the one-shotmechanism which can be also applicable to the continuous distribution.

RAN2 should aim a single solution which offers the flexibility to beapplicable to both use cases of the continuous distribution and theone-shot mechanism.

1. A communication method, comprising: a step of performing, by a userterminal, a reselection of a target cell to be used as a serving cellfrom among a plurality of cells operated at different frequencies,wherein the step of performing the reselection of the target cellcomprises: a first reselection step of performing, by the user terminal,the reselection of the target cell in a response to reception of apaging signal for requesting the reselection of the target cell from acurrent serving cell; and a second reselection step of performing, bythe user terminal, the reselection of the target cell in a response to atrigger without dependence on the paging signal, and in the firstreselection step, the user terminal starts measurement of a quality of ameasurement target frequency included in system information broadcastfrom the current serving cell, in response to the reception of thepaging signal.
 2. The communication method according to claim 1, whereinin the step of performing the reselection of the target cell, the userterminal performs the reselection of the target cell by use of anidentifier of the user terminal.
 3. The communication method accordingto claim 1, wherein in the step of performing the reselection of thetarget cell, the user terminal performs the reselection of the targetcell from a plurality of cells having a quality that satisfies apredetermined quality criteria.
 4. The communication method according toclaim 1, wherein in the first reselection step, the user terminalperforming the measurement of the quality of the measurement targetfrequency even if a quality of the current serving cell satisfies apredetermined quality criteria.
 5. The communication method according toclaim 1, wherein in the first reselection step, the user terminalfurther select the target cell from two or more candidate cells if thetwo or more candidate cells having the same priority are found by thereselection of the target cell in the response to the reception of apaging signal.
 6. A user terminal, comprising a controller configured toperform a reselection of a target cell to be used as a serving cell fromamong a plurality of cells operated at different frequencies, whereinthe controller is configured to: perform a first process of performingthe reselection of the target cell in a response to reception of apaging signal for requesting the reselection of the target cell from acurrent serving cell; and perform a second process of performing thereselection of the target cell in a response to a trigger withoutdependence on the paging signal, and the controller is configured tostart measurement of a quality of a measurement target frequencyincluded in system information broadcast from the current serving cell,in response to the reception of the paging signal.
 7. A processor forcontrolling a user terminal, configured to execute: a process ofperforming a reselection of a target cell to be used as a serving cellfrom among a plurality of cells operated at different frequencies,wherein in the process of performing the reselection of the target cell,the processor is configured to execute: a first process of performingthe reselection of the target cell in a response to reception of apaging signal for requesting the reselection of the target cell from acurrent serving cell; and a second process of performing the reselectionof the target cell in a response to a trigger without dependence on thepaging signal, and in the first process, the processor is configured toexecute of a process of starting measurement of a quality of ameasurement target frequency included in system information broadcastfrom the current serving cell, in response to the reception of thepaging signal.